A missile attack in Syria triggered a blast powerful enough to register as a 2.6-magnitude earthquake and reportedly killed at least 25 pro-government fighters, including some Iranians.
The origin of late Sunday's strike was unclear but there was speculation it was carried out by Israel which has grown increasingly concerned at the buildup of Iranian forces in Syria.
Danny Danon, Israel's ambassador to the U.N., said Friday that more than "80,000 extremists from across the Middle East" are members of Shiite militias in Syria "under Iranian control." Danon also identified what he said was an Iranian "induction and recruitment center" inside the war-torn country.
Israel did not immediately comment Monday, but it has not denied involvement in a similar deadly attack on the T4 air base in Homs earlier this month.
The blast from Sunday night's strike, which hit the western city of Hama, registered as a 2.6-magnitude earthquake, the European-Mediterranean Seismological Centre said.
The U.S.-led military operation fighting the Islamic State in Syria said it was not involved in the attack.
The London-based Syrian Observatory for Human Rights said the strike targeted an arms depot for surface-to-surface missiles at a base known as Brigade 47.
More than 25 Iranian fighters "and their allies" were killed, it said, but the toll could rise as the attack also wounded at least 60 others.
Iran's semi-official ISNA news agency said the Hama strike killed 18 Iranians, including a commander.
Missiles targeted buildings and centers which likely include a weapons depot, ISNA reported, citing unidentified local sources.
However, another Iranian semi-official news agency, Tasnim, denied that Iranian fighters were killed or that Iranian-run bases were hit, citing unidentified Iranian officials.
Lebanese newspaper al-Akhbar reported that the bases housed weapons serving both the Syrian army and the Iranian Revolutionary Guards Corps, and had been attacked with bunker-busting missiles. NBC News could not independently confirm the report.
Israel is determined to prevent Tehran from establishing a presence in Syria, and has grown concerned at the number of Iranian or Iran-backed fighters there supporting President Bashar al-Assad in the seven-year civil war.
Amos Yadlin, a former Israel Defense Forces intelligence chief and director of the Institute for National Security Studies, said he believed Israel was likely behind Sunday's strike and warned that Iran might retaliate.
He highlighted that Israel has "announced again and again that it is determined not to let Iran build their forces in Syria" in recent weeks.
Alastair Jamieson reported from London, Paul Goldman from Tel Aviv, and Charlene Gubash from Cairo.
Israel's Haaretz news site said the timing and magnitude of the attack "may be an attempt to foil any Iranian response to the attack on the T4 air base earlier this month."