UPDATE: 4:23 a.m. EST -- The U.S.-led coalition, which was suspected to have killed four Syrian soldiers in an airstrike in the western Deir ez-Zor province, has denied any involvement in the attack, Al Jazeera reported on Monday.

The airstrike occurred sometime in the last 24 hours in Deir ez-Zor, an ISIS stronghold, according to the Syrian Observatory for Human Rights.

"We did not strike any vehicles or personnel targets in this area. We have no indication any Syrian soldiers were even near our strikes," the U.S. military told Al Jazeera, adding: "We take all allegations of potential collateral damage seriously and will look into every allegation we receive."

The Syrian foreign ministry said Monday that it has sent letters to the United Nations, condemning the attack. The ministry reportedly said that there were four fighter jets from the U.S.-led coalition that targeted a Syrian army camp with nine missiles.

"The Syrian Arab Republic strongly condemns this flagrant aggression by the U.S.-led coalition forces, which blatantly violates the objectives of the U.N. charter," the letters read.

Original story:

Four Syrian soldiers were killed in an airstrike, believed to have been carried out by the U.S.-led coalition against the Islamic State group (ISIS), the Syrian Observatory for Human Rights said Monday. The airstrike occurred sometime in the last 24 hours in the western Deir ez-Zor province, an ISIS stronghold.

If the strike is confirmed, it would be the first to hit Syrian government forces since the coalition began its bombing campaign, Reuters reported. About 13 military personnel were injured in the attack, the U.K.-based monitoring group said, adding that another airstrike in the province believed to be carried out by the coalition overnight killed a woman and her two children.

According to a report by Reuters, a source close to the Syrian government confirmed the news about the airstrike and said that vehicles in the region were destroyed. However, a government source told Ria Novosti that 16 people were injured in the attack.

“The airstrike hit an ammunition dump belonging to the Syrian Army in Deir ez-Zor. According to our information, four soldiers have died and 16 were injured. Two tanks were also damaged. This was the work of the U.S.-led coalition,” the source told Ria Novosti, according to Russia Today.

The latest airstrikes were reported a day after Syrian President Bashar Assad slammed the U.S.-led coalition in an interview to the Sunday Times. “They have no respect for international law and we didn’t ask for their cooperation. If they are ready, we will welcome them. This is not personal,” Assad said.

Deir ez-Zor is reportedly of strategic importance to ISIS due to the region's oil fields. The province also links the de facto capital of ISIS -- Raqqa in northern Syria -- to the region it controls in Iraq. The U.S.-led coalition's airstrikes against the Sunni militant group began in September 2014. In recent weeks, Britain and France have also joined the bombing campaign in Syria.

Russia also started conducting anti-ISIS airstrikes in Syria in September and the following month, the militant group claimed responsibility for downing the Russian Metrojet Flight 9268. The Airbus A321 passenger plane was headed to St. Petersburg from Sharm el-Sheikh resort in Egypt when it crashed, killing all 224 people on board. 

Meanwhile, the West has criticized Russia's actions in Syria as being focused on helping Assad -- a staunch ally of Moscow -- eliminate rebel groups opposed to his regime. Russia has so far denied the allegations.