HELSINKI (Reuters) - U.S. Defense Secretary Jim Mattis said on Monday he backed diplomatic efforts to bring an end to the Syrian civil war, which has claimed hundreds of thousands of lives over more than six years of conflict, as Islamic State militants near defeat.

U.S. Defense Secretary Jim Mattis listens to a question during a joint news conference in New Delhi, India September 26, 2017. REUTERS/Adnan Abidi

Mattis was traveling to Finland to meet with leaders from the Northern Group, a forum for 12 northern European countries, where concern about Russia is growing after it annexed Crimea from Ukraine and gave its backing to eastern Ukraine separatists fighting against the Kiev government.

Russia also supports Syrian President Bashar al-Assad, and with its help Assad has recovered control of swathes of lost Syrian territory over the last two years and appears militarily unassailable.

The United States, however, has supported the opposition to Assad during the Syrian war. Secretary of State Rex Tillerson recently reiterated that Assad should leave power, saying the “reign of the Assad family is coming to an end” and “the only issue is how that should be brought about”.

“Secretary Tillerson engaged strongly with Staffan de Mistura about how do we move what has been going on in Astana, how do we move that over to Geneva where we can actually get the U.N. engaged on the way forward,” Mattis told reporters on a plane en route to Finland.

Russia has backed peace talks in Astana, the capital of Kazakhstan, to end the Syrian civil war. U.N. Special Envoy for Syria Staffan de Mistura announced that stalled peace talks between the Syrian government and still-to-be-united opposition would resume in Geneva on Nov. 28.

However, Russia is also fighting against Islamic State in Syria, at times their goals aligning with the United States, whose forces have helped Kurdish-led militias recapture Raqqa and other parts of northern and eastern Syria from the militants.

With Islamic State’s territory rapidly shrinking, the focus was on defeating it in the final few areas, Mattis said - and avoiding conflict as the gap between Russian and U.S. forces shrinks.

What the U.S. strategy wills after the defeat of Islamic State is likely to come up when Mattis visits Brussels later this week, where he will meet with his counterparts from NATO.

Mattis declined to comment on whether the United States would take back arms from Kurdish YPG fighters after the defeat of Islamic State, a sensitive point given the concerns of its NATO ally Turkey.

Ankara views the YPG as an extension of PKK militants waging an insurgency on Turkish soil, and has sharply criticized U.S. support to the group.