Turkish President Tayyip Erdogan attends the opening session of the World Climate Change Conference 2015 (COP21) at Le Bourget, near Paris, France, November 30, 2015. REUTERS/Christian Hartmann

ANKARA/WASHINGTON (Reuters) - U.S. President Barack Obama and Turkish President Tayyip Erdogan on Tuesday pledged continued cooperation in the fight against militants, including Islamic State and Kurdistan Workers Party (PKK) militants, the White House and Turkish presidential sources said.

During his phone call to Erdogan, Obama offered his condolences for last week’s bombing in Istanbul, when 10 German tourists were killed in a suicide attack blamed on Islamic State, the White House and Turkish presidential sources said.

Obama also condemned a recent string of attacks by the PKK against Turkish security forces, and he stressed the need for de-escalation, the White House said in a statement.

The two leaders said the fight against terrorism would be among a number of topics on the agenda when U.S. Vice President Joe Biden visits Turkey on Saturday.

NATO member Turkey, a member of the U.S.-led coalition battling Islamic State in Syria and Iraq, has increasingly become a target for the Sunni Muslim militants.

Prime Minister Ahmet Davutoglu blamed Islamic State for the bombing on Jan. 12 in Istanbul’s historic heart. The suicide bomber is thought to have crossed recently from Syria.

Islamic State is also believed to be behind other attacks last year in Turkey, including one in the capital Ankara in which more than 100 people were killed.

Turkey’s predominantly Kurdish southeast is currently engulfed in the worst violence since the 1990s after the collapse last July of a two-year-long ceasefire with PKK militants.

Last week the PKK, deemed a terrorist organization by the United States and the European Union as well as by Turkey, attacked a police station in a Diyarbakir district with a truck bomb, killing six people including a baby and two toddlers.