Moscow: Russia has warned the United States against carrying out any attacks on Syrian government forces, saying it would have repercussions across the Middle East.

Russian news agencies quoted Foreign Ministry spokeswoman Maria Zakharova as saying that a US intervention against the Syrian army “will lead to terrible, tectonic consequences not only on the territory of this country but also in the region on the whole”.

She says regime change in Syria would create a vacuum that would be “quickly filled” by “terrorists of all stripes”.

US-Russian tensions over Syria have escalated since the breakdown of a ceasefire last month, with each side blaming the other for its failure. Syrian government forces backed by Russian warplanes have launched a major onslaught on rebel-held parts of the northern city of Aleppo.

On Friday, Russia said it was sending more warplanes to Syria to further ramp up its campaign of air strikes. The Izvestia newspaper reported that a group of Su-24 and Su-34 warplanes had arrived at Syria’s Hmeymim base.

“If need be, the air force group will be (further) built up within two to three days,” it quoted a military official as saying. “Su-25 ground attack fighters designated to be sent to Hmeymim have already been selected in their units and their crews are on standby, awaiting orders from their commanders.”

The Su-25 is an armoured twin-engined jet which was battle-tested in the 1980s during the Soviet war in Afghanistan.

It can be used to strafe targets on the ground, or as a bomber.

Meanwhile, in a statement issued by the White House after the two leaders spoke by telephone, US President Barack Obama and German Chancellor Angela Merkel described the Russian and Syrian bombing of Aleppo as “barbarous”.

Fighting intensified a week into a new Russian-backed government offensive to capture all of Syria’s largest city and crush the last remaining urban stronghold of the rebellion.

Moscow and its ally, Syrian President Bashar Al Assad, spurned a ceasefire this month to launch the offensive, potentially the biggest and most decisive battle in the Syrian civil war which is now in its sixth year.

The Kremlin said on Friday there was no time frame for Russia’s military operation in Syria.

Britain’s Special Representative to Syria, Gareth Bayley, said: “From Russia’s first air strikes in Syria, it has hit civilian areas and increasingly used indiscriminate weapons, including cluster and incendiary munitions.”

“Today, the reality in Syria is a nightmare. Aleppo is besieged again, with vital necessities such as water, fuel, and medicine running out for hundreds of thousands. Civilian infrastructure, including schools and hospitals, are being attacked.”