WASHINGTON (Reuters) - Hundreds of members of the U.S. Congress signed a letter to President Donald Trump on Monday arguing that the United States should remain engaged with the conflict in Syria, saying they were “deeply concerned” about extremist groups in the country.

“As some of our closest allies in the region are being threatened, American leadership and support are as crucial as ever,” said the letter, signed by nearly 400 of the 535 members of the House of Representatives and Senate.

Many U.S. lawmakers, Trump’s fellow Republicans as well as Democrats, have been deeply concerned about Syria policy since December, when Trump confounded his own national security team and allies with a surprise decision to withdraw all 2,000 U.S. troops from Syria.

Trump backtracked in February, agreeing to leave a small U.S. presence to help keep pressure on Islamic State during what the U.S. military believes will be a critical stabilization phase in Syria.

The lead signers of the letter were the Democratic chairman and ranking Republican of the House of Representatives Foreign Affairs Committee, Representatives Eliot Engel and Mike McCaul, and the Republican chairman and ranking Democrat on the Senate Foreign Relations Committee, Senators Jim Risch and Bob Menendez.

Among other things, the letter urges Trump to increase pressure on Iran and Russia with respect to their activities in Syria and increase pressure on the heavily armed Shi’ite Hezbollah movement, Iran’s Lebanese ally.