NEW DELHI, May 10— Sri Lanka's chief censor is an amiable executioner of words. With lips slightly pursed as he pores over articles, he zealously wields his thick black marker even as he shares a chummy exchange with journalists sitting around his desk.

His job is to purge criticism of the government's faltering war on separatist rebels from newspapers, radio and television. ''Sometimes they criticize army officers by name -- by name!'' the censor, Ariya Rubasinghe, said this week during an interview at his office in Colombo, the capital. ''That is nonsense.''

Mr. Rubasinghe is very busy these days scratching out chunks of objectionable copy. The government has recently suffered devastating setbacks in its 17-year war with the Liberation Tigers of Tamil Eelam, and last week it imposed what journalists here said was the harshest censorship of the press ever. It also banned public rallies and trade union strikes that are deemed contrary to national security, as well as ''seditious'' words, defined as those spoken or written to try to create discontent among the citizenry.

Like many governments at war, governments in this democratic nation have periodically clamped down on the press during the long campaign against the rebels, who have frequently used terrorist tactics in a crusade to win a separate state for the Hindu Tamil minority. But the new regulations are more far-reaching and include harsh new penalties. A newspaper's printing presses can be seized, its offices closed and its journalists punished for refusing to submit to Mr. Rubasinghe's pen. The censorship rules are also imposed on foreign correspondents and international news agencies filing articles from Sri Lanka.

During a civil war that has taken more than 60,000 lives and scared away many tourists and much foreign investment in this lovely island nation of 18 million people, the curtailment of civil liberties is one of the less noticed costs. Mr. Rubasinghe, the government's director of information and chief censor -- responsible for supervising a staff of 10 subcensors -- noted how willingly the press has complied with the new rules. ''They are 100 percent cooperative, except a very few,'' he said.

But Iqbal Athas, a veteran defense correspondent for The Sunday Times, said: ''They can arrest me, take my house, my vehicle. They can imprison me for as long as a year without any legal process.''

President Chandrika Kumaratunga has chosen to censor those who have most consistently warned about the shortcomings of the war effort at a time when all the military gains made during her six years in power are in danger of coming undone.

In a speech to the nation on Monday, Mrs. Kumaratunga -- who has been credited with trying to stop the use of torture and extrajudicial killings by the military -- said the emergency regulations were only temporary and would be carried out so ''there will be a minimum of violations of fundamental human rights.''

But opposition leaders accuse her of using the crisis to squash bad news for political reasons. Parliamentary elections are to be held in a few months, and the military reversals can only hurt her party.

The censorship is also unpopular with some voters. While she was on her break, Iromi De Silva, a hotel worker in Colombo, tried to make sense of Mr. Athas's column about the war in The Sunday Times, but she could not follow it. There were large patches of white space where words would have been. A cartoon showed a blindfolded man gagged by a sheaf of emergency regulations.

''We don't get any news by reading this,'' Mrs. De Silva said in disgust. ''And we have a right to know.''

Mrs. Kumaratunga won election in 1994 as a tribune of peace. Her administration spent months negotiating with the Tigers, but the talks broke down, and she decided to try to crush them militarily.

At the cost of thousands of lives, the military recaptured the city of Jaffna, heartland of the Tamil-dominated north, from the Tigers, and has held the city for five years. Now, Jaffna is at risk again; the Tigers have put the government troops on the defensive and taken two crucial military bases. They started another offensive today and claim to have come within two miles of Jaffna.

Diplomats, military analysts and journalists say the government's failings have been those of leadership, of strategy and of faulty, if not corrupt, procurement of the basic tools of war. The war effort has been led by Mrs. Kumaratunga's uncle, Anuruddha Ratwatte, whom she has elevated from a reserve lieutenant colonel to a three-star general.

While the government-owned Daily News has blared headlines in recent days like ''Support President to Safeguard Lanka,'' independent newspapers have been running columns with huge blank spaces stamped with the word ''censored.''

Roy Denish, a 32-year-old correspondent for The Sunday Leader, turned in a 2,200-word column. Most of it was cut out, including a simple description of the military losses and the government's attempt to portray them as part of a tactical withdrawal. Censors also eliminated reporting that the air force's shortage of airworthy aircraft was ''due to bungling by Defense Ministry authorities.''

International news agencies based in Colombo have also been censored. Last Friday, on the first day the new rules were enforced, news agency reporters said their descriptions of an opposition rally called to protest the press restrictions were censored.

Lately Reuters, for instance, has been filing its main daily news story about the conflict out of London.

The New York Times submitted a news article to Mr. Rubasinghe on May 5, in which he made minor changes. This article on censorship was reported in Colombo, but was not censored because it was filed from New Delhi.

In Sri Lankan newspapers, which have been inserting the word ''censored'' in places where cuts have been made, some deletions have a kind of teasing quality. A column in The Island about India's role in the conflict, written by a retired Sri Lankan Foreign Ministry official, K. Godage, included these snippets: ''Sri Lanka is in dire straits -- CENSORED,'' and ''My question to Indian authorities is why they wish to wash their hands off CENSORED.''

But even Sri Lankan journalists who have been heavily censored by Mr. Rubasinghe speak fondly of him. For six years, he has been the man they called for the government's version of events, and he has always been pleasant.

''Even when he does the most painful things, you can't get mad at him,'' said Mr. Athas, 55, the country's leading military correspondent.

But Mr. Athas is a keen critic of censorship and of the government's management of the war. In recent months his readers have been given presentiments of the government's disastrous military performance.

His readers also know the lengths to which some people in the military have gone to intimidate him. Two air force officers have been charged with breaking into his home two years ago, holding an automatic pistol to his head and terrorizing his wife and 7-year-old daughter.

At the time, Mr. Athas had been writing exposes about the flawed decisions of the air force and irregularities in its purchases of aircraft. ''The moral, it seems, is do not expose corruption or misdeeds,'' he wrote then. And now the government has explicitly forbidden journalists to write about the military procurement process.

Mr. Athas said the government seems to have convinced itself, ''If a story isn't told, it hasn't happened.''

Photo: What censorship looks like: military news was heavily edited in an independent newspaper, The Sunday Times, in Colombo, Sri Lanka. (Associated Press)