Syria expressed outrage Monday after a suspected US-led coalition strike for the first time killed regime troops, but the coalition denied its warplanes hit an army base.

In a letter to the United Nations Security Council and Secretary General Ban Ki-moon, Syria's foreign ministry condemned what it called a "flagrant aggression" that killed at least three soldiers late Sunday.

But a spokesman for the US-led coalition said its only strikes in the area on Sunday were some 55 kilometres (35 miles) southeast of the Syrian army base.

"The Syrian Arab Republic strongly condemns this flagrant aggression by the US-led coalition forces, which blatantly violates the objectives of the UN charter," the foreign ministry said in the letter.

It demanded the Council "act immediately in the face of this aggression and take appropriate measures to prevent its recurrence."

It said three Syrian soldiers were killed and 13 wounded in strikes by four coalition planes on an army camp in the eastern province of Deir Ezzor.

A Syrian military source and the Syrian Observatory for Human Rights, a Britain-based monitor, said four soldiers had been killed and 13 wounded in the strike, near the town of Ayyash.

The military source said the attack happened Sunday night and hit several buildings used as weapons depots and an army training camp, damaging two tanks.

- 'No evidence' -

The Observatory said it was the first time a US-led coalition strike had killed Syrian government troops.

Much of Deir Ezzor is under the control of the Islamic State group, which the US-led coalition regularly targets in the province.

However, the regime remains present in small areas, including in the provincial capital.

The province's oil resources have been a major source of IS funding, but on Monday analysis firm IHS said the group was having trouble making ends meet due to air strikes on its oil infrastructure.

IHS estimated the extremist group's overall monthly income to be about $80 million (75 million euros) as of late 2015, around half of it from levies and confiscations.

But it noted the group also had significant costs because it administers large swathes of territory.

The US-led coalition began strikes in Syria in September 2014, expanding a campaign against IS that began in neighbouring Iraq.

A coalition spokesman Monday denied it was behind the alleged strikes, saying its warplanes carried out no raids in the area Sunday.

"We've seen those Syrian reports but we did not conduct any strikes in that part of Deir Ezzor yesterday. So we see no evidence," Colonel Steve Warren told AFP.

"We struck 55 kilometres away from the area that the Syrians say was struck. That was the only area in Deir Ezzor we struck yesterday," he added.

"There were no human beings in the area that we struck yesterday, all we struck was a wellhead."

- Coalition 'not serious': Syria -

The Syrian government has regularly criticised the US-led strikes as ineffective and illegal because they are not coordinated with regime forces, and the foreign ministry said the Deir Ezzor incident was further evidence of the coalition's failings.

"The US coalition lacks the seriousness and credibility to effectively combat terrorism," the ministry said.

Staunch regime ally Moscow began its own aerial campaign in Syria on September 30 and coordinates its air strikes with government troops.

On Sunday, US President Barack Obama vowed to destroy IS and hunt down its followers at home and abroad in a rare address from the Oval Office.

The speech followed a shooting rampage in California last week that saw an apparently radicalised couple kill 14 people.

While pledging to "hunt down terrorist plotters in any country", Obama also said he would not be "drawn once more into a long and costly ground war in Iraq and Syria," as sought by IS.

"They know they can't defeat us on the battlefield... but they also know that if we occupy foreign lands, they can maintain insurgencies for years, killing thousands of our troops and draining our resources, and using our presence to draw new recruits," he said.

Elsewhere, Syrian media said three people were killed and five wounded in rebel rocket fire that hit near the now-closed Russian consulate in Aleppo city.

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IHS Global Insight