BEIRUT (NYTIMES) - Israel on Monday (April 9) appeared to have escalated its shadow war in Syria against Iran, with a pre-dawn air strike against a military base that coordinates Iranian-backed militias, killing four Iranian military advisers.

The dead included a colonel who served as a senior officer in Iran's drone programme, according to Iranian news reports.

The attack on the Syrian air base near the desert town of Palmyra in central Syria drew new attention to a conflict between Iran and Israel that has been steadily increasing in intensity while mostly hidden in the shadows of Syria's civil war.

As Iran has taken advantage of the war's chaos to build substantial military infrastructure, Israel has launched scores of strikes to try to stop it, or at least to slow it down.

Israeli officials declined to confirm or deny that Israel had conducted the air strike. It followed a vow by President Donald Trump to respond to an apparent chemical weapons attack by the Syrian government near Damascus on Saturday.

It did not appear to have been in response to that event, raising the possibility that Israel had merely seized the opportunity to take out what it saw as a threat to its own security.

Russian and Iranian news services reported that two Israeli F-15 war planes carried out the strike, which a conflict monitor said killed 14 people.

The Russian military said the planes had approached from the Mediterranean Sea before firing from Lebanese airspace.

Syria's air defence systems shot down five of the eight missiles fired, according to the Russian news agency Interfax.

The other three missiles hit a Syrian military base known as T4 that has played a central role in Iran's expanding military activities in Syria.

Iran has long been a close ally of Syrian President Bashar al-Assad.

As the war in Syria ground on, Iran came to Mr Assad's aid, sending seasoned fighters from Hizbollah, the Lebanese militant group and political party that Iran supports.

Israel now worries that as Mr Assad's position becomes more secure, Iran has turned its focus to the military capacity it appears to be building to help it in a future confrontation with Israel.