By Sarah Rainsford BBC News, Istanbul

Turkey's military said it had achieved its goals in northern Iraq

The timing of the Turkish military withdrawal from northern Iraq came as a surprise.

Just hours after US President George W Bush called on Turkey to end its operation against the separatist Kurdistan Workers' Party (PKK) as soon as possible, troops were rolling back across the border.

The first clear signs of the withdrawal came midway through Friday morning, when a private local television channel reported dozens of army trucks ferrying tired-looking commandos back to their bases in south-eastern Turkey.

NTV reported that many more empty trucks were heading towards the border, to collect their comrades.

It was several hours before the Turkish General Staff officially confirmed the complete withdrawal on its website.

The statement was adamant that the move was pre-planned - judged according to military need, not forced through political pressure.

But the decision has baffled many Turkish commentators.

'Real blunder'

On a visit to Ankara on Thursday, US Defence Secretary Robert Gates delivered the message that Washington's support for this operation was not open-ended.

Politicians here - even Turkey's chief of staff - responded with defiance. Their message: that Turkey had set no timetable for the operation and troops would stay inside northern Iraq until its goals had been met.

People will say the US gives orders and we obey - that might not be the truth, but it's the perception Sedat Laciner, International Strategic Research Organisation

"Such a prompt end to this operation just hours after those statements creates real question marks. Everyone here wants to know why," says Cengiz Candar, a columnist with the daily newspaper, Referans.

"This will be seen as a real blunder. It's a real credibility issue for the government and for the military," he adds, speculating that perhaps the US issued a far harsher warning to Turkey behind closed doors than in public.

American support was crucial to the cross-border operation. The US opened Iraqi airspace to Turkish fighter planes last December and has been supplying intelligence from inside northern Iraq ever since.

Officially though, the Turkish military says it ended this operation because its objectives had been reached.

"There was no question of liquidating the terrorist organisation completely," its statement said. "But Turkey has shown the PKK that northern Iraq is not a safe-haven."

'Limited scope'

The military claimed 240 PKK fighters and 27 members of the Turkish security forces were killed in just over a week of fighting and almost 800 shelters, weapons stores and other PKK positions were destroyed.

The PKK was formed to carve a separate homeland for the Kurds

The focus of the fighting was the Zap Valley - portrayed here as a PKK stronghold.

"Within the limited scope of this operation, this is a success story," believes Sedat Laciner, the director of the Ankara-based think tank, the International Strategic Research Organisation (USAK).

"But there will be major public disappointment that the incursion did not last as long as people thought. It's been badly managed. People will say the US gives orders and we obey. That might not be the truth, but it's the perception."

Even as the first military vehicles were bringing troops back across the border, the morning newspapers were trumpeting Turkey's challenge to the world.

"We'll come back when the job's done!" was one typical headline.

The actual military impact of the operation is unlikely to be clear until the spring, when the mountain snow melts, and the PKK traditionally launches attacks.

Turkey has always said there are more than 3,000 PKK fighters inside northern Iraq. The military statement underlines that it will continue to keep a close eye on their activity and hints that troops will go back in after them, if necessary.

"The fight against terror will continue with determination at home and abroad," it added.

'Kurdish issue'

But the US has delivered a second message to Turkey in recent days - that the government must tackle the roots of support for the PKK on its side of the border.

The PKK was formed in the 1980s to carve a separate homeland for the Kurds from a country that then refused to acknowledge they even existed. Today, it claims it is fighting for greater democratic rights for the Kurds inside Turkey.

The Kurds' situation has improved immensely in recent years, but considerable problems remain.

"You can't defeat the PKK with military might alone," believes Mr Laciner.

"Even if you kill thousands of terrorists, then inside Turkey itself, in Iran or Syria - or in the Kurdish diaspora in Europe - hundreds more will join the PKK. There's a social and political dimension to this too."

But despite numerous promises from the government to tackle the "Kurdish issue" at home, there has been little concrete progress since a raft of EU accession- inspired reforms several years ago.

"It might happen now," muses Mr Candar, but he sounds doubtful. He argues there's another pressing issue for Turkey to attend to.