President Barack Obama plans to send as many as 250 more US troops to Syria, bringing the total American presence on the ground to 300 to help fight Islamic State militants, US officials say.

The decision, which a US official says will be announced in Hanover, Germany, on Monday, was first reported by the Wall Street Journal and confirmed an April 1 Reuters report that the Obama administration was considering a significant increase in US forces.

The additional deployment aims to accelerate recent gains against Islamic State and appears to reflect growing confidence in the ability of US-backed forces inside Syria and Iraq to claw back territory from the hardline Sunni Islamist group.

Islamic State, also known as ISIS or ISIL, controls the cities of Mosul in Iraq and Raqqa in Syria and is proving a potent threat abroad, claiming credit for major attacks in Paris in November and Brussels in March.

While Obama has resisted putting US troops into Syria, where a five-year civil war has killed at least 250,000 people, he sent 50 US special operations forces to Syria last year in what US officials described as a "counterterrorism" mission rather than an effort to tip the scales in the war.

"He (Obama) intends to put in more ... forces to the tune of 250 in Syria," said one US official, adding he was unable to break down how many of those would be special operations forces and how many might be medical or intelligence support personnel.

"The president has authorised a series of steps to increase support for our partners in the region, including Iraqi security forces as well as local Syrian forces who are taking the fight to ISIL," said a second Obama administration official.

Obama will announce the latest deployment during his speech at the Hanover Messe fairgrounds on Monday, that official said.

There are mounting indications the momentum in Iraq and Syria may have shifted against Islamic State.

In Iraq, the group has been pulling back since December when it lost Ramadi, the capital of the western province of Anbar. In Syria, the jihadist fighters have been pushed from the strategic city of Palmyra by Russian-backed Syrian government forces.

Since US-backed forces recaptured the strategic Syrian town of al-Shadadi in late February, a growing number of Arab fighters in Syria have offered to join the fight against the group, US officials told Reuters in early April.

Obama is in Hanover meeting German Chancellor Angela Merkel.

On Monday, the two will be joined by British Prime Minister David Cameron, French President Francois Hollande and Italian Prime Minister Matteo Renzi to discuss Syria and other foreign policy issues.