Gordon Brown has said he "absolutely and unequivocally" condemns Kurdish terrorist attacks on Turkey.
Speaking in Number 10 following talks, Turkish leader Recep Tayyip Erdogan called for action to tackle terrorist organisations based in northern Iraq.
The Kurdistan Workers' Party (PKK) has been blamed for recent attacks that have left more than 40 Turkish soldiers and civilians dead.
Eight troops are still missing after an attack on Sunday left 12 dead, and demonstrations in Turkey have put pressure on the government to launch raids on rebel positions in Iraq.
The British prime minister spoke out against the PKK's activities and pledged to co-operate with the Ankara government.
"We are committed to working with the Turkish government on counter-terrorism activity," Brown said.
The UK would work for a "diplomatic solution" to the crisis affecting northern Iraq, he added.
"We will work hard with the Turkish government in trying to help them in this difficult situation with which they are faced."
Brown said the UK would seek to prevent terrorists moving from Iraq into Turkey and also welcomed the "restraint" shown by Erdogan's administration.
"We will work together to deal with any terrorist atrocities that exist. None can be in any way justified," the prime minister said.
Erdogan said Turkey had no wish to undermine the territorial integrity of Iraq or damage moves towards democracy in the country.
Brown also said the UK would continue to support Turkish membership of the European Union.
The prime minister said he was confident that "this can move forward over the next few weeks and months".
And the Turkish leader said there was a "common vision" for shared relations between the UK and Turkey.
Dods Parliamentary Communications Ltd