Image copyright AFP/getty Image caption The Pentagon intends to train and arm more than 15,000 rebels over the next three years

The US military has begun training a small group of Syrian rebels in an effort to build a force capable of defeating Islamic State militants.

Ashton Carter, US defence secretary, said about 90 people were being trained in a secure location, and they would be paid by the United States.

He said more fighters would begin training soon at sites in Jordan, Turkey, Qatar and Saudi Arabia.

Supporting the troops militarily on the battlefield has not been ruled out.

Nearly 4,000 fighters volunteered for the programme but, after an extensive screening process, only 400 were given places.

'International law'

"We're starting with the people that we have that we've vetted very carefully," Mr Carter said, "We expect that to be successful and therefore to grow. But you have to start somewhere, and this is where we're starting."

Mr Carter was at pains to point out that the role of the force would be to take on the Islamic State and not the regime of President Bashar al-Assad. However, he did not say what would happen if the rebels engaged the Syrian army.

Image copyright Reuters Image caption Moderate rebels have been fighting both Islamic State forces and those of President Assad

"We have not determined yet all of the rules of engagement... but we have acknowledged that we have some responsibility to support them," the defence secretary said.

The fighters will be paid by the United States, and, when deployed, will continue to receive back up and support from coalition forces.

There are about 450 coalition forces involved in the training at the four sites, including about 350 Americans.

The Pentagon intends to train and arm more than 15,000 rebels over the next three years.

Mr Carter made it clear that the support did come with strict conditions.

"An explicit part of their training is how to conduct themselves in a way that is consistent with international law," adding: "Any continued support for them would be strongly conditioned upon their continued good conduct."

The Pentagon would not disclose when the troops would be ready to enter the battlefield.

Since last year, the US has been part of an international coalition conducting an air strike campaign against Islamic State, which took over large swathes of Syria and Iraq in a lightning offensive.