File Photo: Smoke rises after a U.S. air strike on Islamic State terrorist group in Kobani, Syria, Oct. 11, 2014. (Xinhua/Cihan)

WASHINGTON, April 9 (Xinhua) -- The U.S. Department of Defense (DOD) tweeted on Monday that it was not carrying out air strikes in Syria, but that it continues "to support the ongoing diplomatic efforts to hold those who use chemical weapons, in Syria and otherwise, accountable."

The Syrian army said Israeli F-15 warplanes had carried out an attack on the T-4 air base in central Syria by firing missiles from the Lebanese airspace before daybreak Monday, according to state TV.

The attack was initially thought to be carried out by the United States, which had threatened retaliation against the Syrian army over the rebels' allegations of the use of chlorine gas during a Saturday attack in the Douma district, the last rebel-held area in Eastern Ghouta.

U.S. President Donald Trump on Sunday condemned Syria, Russia and Iran for what he called a "SICK" attack, threatening a "big price to pay."

The unexpected rhetoric has sparked concerns over the prospect of a U.S. withdrawal of troops from Syria that Trump had reiterated recently.