Defence Secretary Sir Michael Fallon said the Syrian forces were being trained in infantry, medical and explosive hazard awareness skills.
He spoke ahead of a London summit with defence ministers from the coalition of countries fighting IS, also known as Daesh, in Iraq and Syria.
It comes after a ceasefire between Syrian rebels and the Russian-backed Bashar Assad regime in Aleppo broke down, threatening plans to evacuate civilians amid reports of pro-government troops executing non-combatants.
At the meeting, Sir Michael will reveal that he has agreed to “surge” the number of data recovery experts to exploit material obtained as IS forces are defeated or flee from Mosul in Iraq.
Britain’s top commander in the region, Major General Rupert Jones, said recently that plans revealing thousands of IS plots to attack Europe were discovered after the terrorists were driven out of Manbij, northern Syria.
Sir Michael hopes the extra experts will be able to exploit data and technical equipment seized from IS to help track UK militants, win the battle on the ground, better understand the militants’ structure and leadership, and build a case against fighters who have committed atrocities.
The summit will also mark US Defence Secretary Ash Carter’s last official visit to the UK before he is replaced by Donald Trump’s appointee General James “Mad Dog” Mattis.
Ahead of the summit, Sir Michael said: “Daesh is being taken on in eastern Mosul: last week we opened up a second front around Raqqa. Daesh is losing ground, finance, and fighters.
“As part of the 68-member coalition, Britain is playing a leading role, through our air strikes, intelligence, surveillance and reconnaissance, and our training.
“In 2017 we must maintain momentum to deal these terrorists a decisive blow.”
In recent days RAF jets have bombed IS buildings, positions and equipment around Mosul, while the coalition has carried out more than 300 air strikes in support of the Syrian Democratic Forces’ operation in Raqqa, where the UK has hit weapons factories, IS strongholds and anti-aircraft guns.
The RAF is working more intensely on a single operation than at any point in 25 years, the Ministry of Defence said.
Warplanes flew almost 14,000 hours over the last year, compared to 5,600 during the most intense 12 months in Afghanistan.
Since June, over 70% of UK air strikes have been in support of the Iraqi advance in Mosul.
In addition, 80 engineers from 22 Engineer Regiment are to extend their tour of duty by six months in order to carry out upgrades to the Al Asad air base in Iraq, where UK soldiers are training Iraqi forces.