The military said 77 separatist Kurdistan Workers' Party (PKK) militants were killed overnight in "the heaviest clashes" since its forces rolled over the border into the snow-bound mountains of northern Iraq last week.

That brought the army toll of PKK dead to 230, while its own losses climbed to 27 with the deaths since Tuesday evening of five soldiers and three government-armed Turkish Kurd militiamen helping the army.

As fighter jets continued to pound rebel positions on the sixth full day of the incursion, US Defense Secretary Robert Gates made it clear that US support for its NATO ally was not open-ended.

The offensive must end quickly, he said.

"I measure quick in terms of days, or a week or two, something like that. Not months," he said in New Delhi before flying to Ankara for talks.

But a senior Turkish official said the operation would continue until the PKK, listed as a terrorist group by much of the international community, is uprooted from the region, which it uses a springboard for attacks in Turkey.

"There will be no timetable to withdraw Turkish troops... until the presence of the terrorist organisation is eliminated," said Ahmet Davutoglu, chief foreign policy advisor to Prime Minister Recep Tayyip Erdogan.

A delegation led by Davutoglu was dispatched to Baghdad as Ankara sought to allay mounting Iraqi protests over the incursion.

"We condemn terrorism. We condemn the PKK but at the same time we condemn any violation of Iraqi sovereignty," Iraqi Foreign Minister Hoshyar Zebari said after talks with the delegation.

The United States has backed the offensive, supplying Turkey with intelligence on PKK movements, and said the Turkish army had acted "responsibly so far."

But Washington is wary of the prospect of conflict between Turkish forces and the Kurdish administrators of northern Iraq -- two key US allies with chilly ties.

Turkey has long accused the Iraqi Kurds of providing the PKK with a safe haven and weapons. The military warned the Iraqi Kurds this week not to shelter PKK rebels fleeing the fighting.

A senior US official travelling with Gates to Ankara said they had debated whether to drop Ankara from the secretary's schedule as a message to the Turks.

"After a short discussion, everybody decided it was best to go and engage the Turks personally," the official said on the condition of anonymity.

Gates' biggest lever over Turkey is access to US intelligence.

A US decision to suspend intelligence sharing would be a significant loss for Turkey and could result in operations that were less precise.

Gates urged Ankara to back up military action with political and economic measures that would erode popular Kurdish support for the PKK.

"They need to deal with some of the issues and complaints that some of the Kurds have and move this in a non-military direction in order to get a long-term solution," he said.

Erdogan is already under pressure at home to improve Kurdish rights, tackle rampant poverty in the Kurdish-majority southeast and issue an amnesty for PKK rebels to encourage them to end their armed separatist campaign.

Gates will meet Erdogan, President Abdullah Gul, Defence Minister Vecdi Gonul and Chief of General Staff Yasar Buyukanit Thursday.

The Turkish army said warplanes and artillery maintained fire on rebel positions and hide-outs "deep" in the operation area Wednesday.

PKK leaders are believed to be in groups involved in fighting on the ground, it said.

The PKK claims to have killed around 90 soldiers, including five in an ambush Tuesday, and to have downed a Turkish attack helicopter.

Ankara estimates some 4,000 rebels are holed up in northern Iraq.

The PKK took up arms for self-rule in southeast Turkey in 1984, sparking a conflict that has claimed more than 37,000 lives.