Syria's use of Scud missiles against rebels, as claimed by Washington and rebel fighters, is a desperate move by a regime that has exhausted its military capacity, analysts say.

A US official said on Wednesday that the regime had fired Scuds, and a former Syrian officer who served in a battalion specialising in surface-to-surface missiles also claimed troops had done so.

Damascus denied that it had used the missiles.

Karim Bitar, research director at the Institute for International and Strategic Relations, said: "Scuds are cumbersome, inaccurate and very expensive and there is no military justification for using them.

"Their use is therefore clearly part of the regime's psychological war against the rebels and countries that support them," he said.

Bitar said using Scuds was an indication that the regime was bracing for a decisive battle in Damascus, which "could change the rules of the game."

Riad Kahwaji, founder of the Institute for Near East and Gulf Military Analysis (INEGMA), said Scud missiles "are weapons of terror."

"They are used to make up for an inability to control the ground."

Scuds were last used by Libya's regime in the final days before Moamer Kadhafi was killed, said Kahwaji, referring to the 2011 NATO-backed rebel offensive against the longtime strongman.

"Here we are seeing a similar scenario," said Kahwaji. "It just shows the level of desperation. The regime has exhausted its military capabilities.

"It is like somebody with different size hammers. You use a small hammer first and it doesn't work, so you use a bigger one till you reach the sledge-hammer."

Kahwaji said he believed the regime had a sizeable store of such weapons in army-controlled locations between Damascus and the Alawite heartland of Latakia on the coast.

Former first lieutenant Aaraba Idriss said he was still in contact with officers and members of his former Battalion 57, part of Brigade 155, despite defecting 10 months ago.

He said they told him they fired five Scud missiles for the first time on Monday from their location in Nasiriyeh on the highway between Damascus and the central Syrian city of Homs.

Idriss said the "Golan-1" missiles were either Russian-made or Russian modified" and had a range of up to 300 kilometres (180 miles).

The Syrian foreign ministry categorically denied those claims, calling them "biased and conspiratorial rumours."

"It is known that Scuds are strategic, long-range missiles and are not suited for use against armed terrorist gangs," the foreign ministry said on Thursday.

However, a security source in Damascus said on condition of anonymity on Thursday that the army had used a smaller, Syrian-made version of the Scud.

These developments come as the conflict, which has reportedly claimed more than 42,000 lives, enters its 22nd month.

In recent weeks, rebels have captured a series of key army bases and consolidated their grip on large swathes of north and east Syria.

This has prompted Syria's long-time ally Russia to admit on Thursday that the increasingly bloody conflict in Syria might culminate in a rebel victory.

An AFP correspondent in northwestern Syria on Wednesday witnessed what rebels said was the impact of a Scud, one of six missiles that hit around the Sheikh Suleiman army base, which they captured earlier this month.

"There were 21 olive trees here, now they're all gone," said the owner of the land where one of the missiles struck just outside the town of Darret Ezza.

No one was killed but the missiles shook the town, breaking windows and sparking fears of more attacks.