Turkish forces crossed into Iraq Thursday to strike at Kurdish rebels and warplanes pounded their bases in retaliation for the death of 24 soldiers, officials said.

"A large-scale land operation, backed by air strikes, has begun in five separate spots inside Turkey and across the border with 22 battalions," the Turkish military said in a statement posted on its website.

The 22 battalions comprise commando units as well as gendarmerie and special forces, it added, without specifying how many had entered Iraq. Analysts said a total of 10-15,000 troops would be deployed.

"The air and land operation is under way," Prime Minister Recep Tayyip Erdogan told reporters right after the military announcement.

"The operation is result-oriented," he said, without elaborating.

The separatist Kurdistan Workers' Party (PKK), through spokesman Dozdar Hammo, reacted defiantly, saying: "If they want to come, let them come. We will welcome them here."

Hammo said no Turkish troops had yet crossed the border into northern Iraq, but said Ankara's jets were flying overhead.

The Turkish air force kept up bombing raids overnight in response to Wednesday's coordinated attacks by PKK guerrillas on military posts in Turkey, which caused the worst loss of life for the army since 1993, local security sources said.

According to press reports, between 200 and 250 Kurdish rebels entrenched in the mountains of northern Iraq, crossed into Turkey late Tuesday to carry out raids which left 24 Turkish soldiers dead and 18 wounded.

The United States voiced support for Turkey actions. "We very clearly support Turkey's right to self-defense," State Department spokesman Mark Toner told reporters.

At the same time he urged cooperation between Turkey and Iraq through the committee set up in 2008 by Ankara, Baghdad and Washington to take up the issue of the PKK, long an irritant in Turkey's ties with Iraq and by extension the United States.

Earlier Thursday, a military ceremony was held in Van, a city in eastern Turkey 150 kilometres (90 miles) north of Wednesday's combat zone.

The coffins, draped with the red and white flag of Turkey, were loaded into military aircraft to be taken to their home towns for burial.

President Abdullah Gul, Prime Minister Erdogan as well as several Cabinet ministers and opposition party leaders attended Thursday the funeral in Ankara of one of the fallen soldiers.

The latest attacks sparked widespread outrage throughout the country.

Thousands of people, many of them students, visited the mausoleum of Turkey's founder Mustafa Kemal Ataturk in Ankara and denounced terrorism.

In Istanbul, some 500 people including members of several trade unions took to street and shouted "Turkey is Turkish and will remain so."

Unidentified individuals in rage of the attacks assaulted the offices of the pro-Kurdish Peace and Democracy Party (BDP) in four different provinces on Wednesday night and Thursday, causing material damage, according to the press office of the party.

Turkey's parliament began discussing further measures against the PKK in a closed doors session Thursday.

Iraq in an official statement on Thursday pledged to cooperate with Ankara on security issues.

"The Iraqi government condemns this terrorist activity by the PKK, and expresses its sympathy for the families of the Turkish soldiers," the foreign ministry said in a statement.

It continued: "(Iraq) is committed to collaborate with the Turkish government on security issues to prevent a repeat of such actions."

Erdogan also had a telephone conversation with Massoud Barzani, the president of Iraq's autonomous Kurdistan region, media reported. He is expected to visit Turkey soon.

Ankara has repeatedly urged Baghdad not to allow its territory to be used as a springboard by the PKK for attacks on Turkey.

Nechirvan Barzani, a former prime minister of the Kurdish regional government, who paid a surprise visit to Ankara, said: "We strongly condemn this attack," after meeting Turkey's Foreign Minister Ahmet Davutoglu Thursday.

He also met with Erdogan, who was accompanied by the intelligence chief of Turkey, according to Anatolia news agency.

Iranian Foreign Minister Ali Akbar Salehi will come to Turkey on Friday in a surprise visit to discuss "regional issues and terrorism," a Turkish diplomat told AFP on Thursday.

Since July Tehran has been carrying out a major offensive against the Party of Free Life of Kurdistan (PJAK), which Turkey considers as a branch of the PKK.

Clashes between the PKK and the army have escalated since the summer.

Five police and four civilians were killed in a landmine explosion in the southeast on Tuesday.

The PKK, listed as a terrorist organisation by Turkey and much of the international community, took up arms in Kurdish-majority southeastern Turkey in 1984, sparking a conflict that has claimed about 45,000 lives.