WASHINGTON (AFP) - The United States will welcome 10,000 Syrian refugees this fiscal year as promised by President Barack Obama, Secretary of State John Kerry announced Tuesday (July 12).

Washington has been criticised by some activists for moving too slowly to settle those fleeing the conflict, while Obama's opponents warn their number may include terrorists.

But Kerry said the United States is now on course to admit 10,000 vulnerable refugees, chosen from UN camps and vetted by US security and intelligence agencies.

"It's also representing six-fold increase over what we did the year before," Kerry said, referring to the US fiscal year, which runs from October 1 to September 30.

"I'm proud to say that the United States is by far the largest contributor of emergency aid, but we all recognize that still more needs to be done," he said.

Kerry made the remarks at a dinner in Washington to mark the Muslim holiday of Eid al-Fitr, shortly before he was due to set off on a tour of European capitals.

This journey was to take him Friday to Moscow, where he is set to meet with President Vladimir Putin and lobby Russia to do more to help end Syria's five-year-old conflict.

Syria is in the grip of what Kerry called the worst humanitarian crisis since World War II, and more than 4.8 million people have fled the country.

The United States has traditionally been by far the world's most generous host for refugees but has been criticised for moving too slowly to respond to the Syrian crisis.

Frontline states like Lebanon and Jordan meanwhile have been all but overwhelmed, and the arrival of streams of unvetted migrants on Europe's shores provoked a crisis.

Republican White House hopeful Donald Trump has repeatedly alleged Obama plans allow hundreds of thousands of Syrians to come to the United States without security checks.

But the 10,000 who will be allowed in before Sept 30 were selected in UN camps as vulnerable - such as widows, the elderly and disabled - and screened by US officials.