BEIRUT (Reuters) - A Kurdish militia that has been backed by U.S.-led airstrikes in Syria has joined a new military alliance that includes Arab groups, a statement released overnight said.

The alliance calling itself the Democratic Forces of Syria includes the Syrian Kurdish YPG militia and Syrian Arab groups that have already fought alongside it against the Islamic State militant group in northern Syria.

It comes soon after the United States announced a shake-up of its support to Syrian rebels fighting ISIS, effectively ending its program to train fighters outside Syria and focus instead on providing weapons to groups whose commanders have been U.S.-vetted.

The YPG has to date proved the most effective partner on the ground for U.S.-led airstrikes against Islamic State group, taking large amounts of territory from the jihadists in northeast Syria this year.

"The sensitive stage our country Syria is going through and rapid developments on the military and political front ... require that there be a united national military force for all Syrians, joining Kurds, Arabs, Syriacs and others groups," said the statement, which was sent to Reuters by a YPG spokesman.

The Democratic Forces of Syria includes the YPG, various Arab groups including Jaysh al-Thuwwar (Army of Rebels), and an Assyrian Christian group.