ISTANBUL - TURKISH Prime Minister Recep Tayyip Erdogan is coming under intense pressure to order an invasion of northern Iraq following the deadliest attacks in more than a decade on the Turkish military and civilians by separatist Kurdish guerillas.

Thirteen soldiers were killed in the south-east on Sunday when PKK guerillas outgunned a Turkish unit of 18 men without sustaining any casualties, while on Monday, two soldiers were killed by booby-trap PKK explosions.

Last week, in an ambush also ascribed to the PKK, gunmen sprayed a bus with automatic fire in the same region, killing 13 civilians, including a boy of seven.

The Turkish media described the toll from the attacks as the worst in 12 years, according to a report in yesterday's Guardian newspaper.

Rebels from the PKK, or the Kurdish Workers' Party, have been classified as terrorists by Ankara, Washington and the European Union. The party has been waging a bloody campaign for self-rule in Turkey's mainly Kurdish east and south-east since 1984.

Mr Erdogan has so far heeded strong US opposition to any invasion moves, despite demands over the past six months from the Turkish armed forces for a green light to cross the border into Iraqi Kurdistan, where the guerillas are based.

Washington says an incursion would immensely complicate its own campaign in Iraq and destabilise the only part of Iraq that is functioning - the Kurdish-controlled north.

Mr Erdogan is also known to think little of the invasion option, believing it would probably fail, said The Guardian.

The Turkish military raided Iraqi Kurdistan dozens of times in the 1990s but was unable to suppress the insurgency.

But on Monday, the Premier called an emergency meeting of national security chiefs to ponder their options in the crisis.

'What is at issue here is how much any action we decide to take would bring us closer to a result,' Turkish government spokesman Cemil Cicek said afterwards. He did not rule out an invasion, but queried its 'usefulness', the newspaper said.

The Prime Minister, however, is being challenged by the army command, which earlier this year demanded his authority to invade. And he is vulnerable to mounting public clamour to act because of the upsurge in guerilla activity and the heavy casualties inflicted.

Following the soldiers' deaths on Sunday, Mr Erdogan signalled a shift in policy without specifying how. 'Our campaign against terrorism will continue in a different manner,' he said.

The Turkish military has just declared 27 'security zones' on its borders with Iraq and Iran off-limits to civilians, suggesting to some that it might be gearing up for an invasion, said The Guardian.

But despite the rising violence, Mr Erdogan has opted for politics in his attempts to defuse the conflict.

His Justice and Development party (AKP) enjoyed a stunning success among the Kurdish minority - concentrated in the country's south-east - in the July elections and he has also focused on political pacts with Baghdad to get the better of the guerillas.

Last week, Iraqi and Turkish interior ministers signed a deal aimed at combating the PKK by trying to cut the rebels' funding and logistics, and agreeing to extradite captured 'terrorists'.

The accord, however, took three days to thrash out; Turkish insistence on allowing cross-border raids was denied and there is scepticism over whether Baghdad can deliver.

Officially, Ankara refuses to recognise or deal with the government of Iraqi Kurdistan, although there have been back-channel attempts over the past year to engage Mr Massoud Barzani, President of the Iraqi Kurdish region.

Mr Erdogan's options are also constrained by strong US hostility to an invasion.

While Turkish public opinion has been strongly anti-American since the 2003 invasion of Iraq, much of the logistical support for the US troops goes to Iraq via Turkey.

Relations are also under severe strain because of US congressional moves to brand the 1915 massacres of Armenians in Ottoman Turkey as 'genocide'.