By Tom Perry and Suleiman Al-Khalidi

BEIRUT/AMMAN (Reuters) - Syrian rebels launched an attack on a town held by Islamic State near the Iraqi border on Tuesday, senior commanders said, opening a new front against militants also facing an offensive from U.S.-backed forces in the north.

If the rebel New Syria Army succeeds in capturing the town of Al-Bukamal from Islamic State (IS), it would hamper the jihadists' ability to move between the Iraqi and Syrian territories of their self-declared caliphate.

Armed and equipped by the U.S.-led coalition, the New Syria Army was formed 18 months ago from Arab rebels driven out of eastern Syria by Islamic State as it expanded its territories in mid-2014 following the capture of the Iraqi city of Mosul.

A commander said the rebels had secured the desert approaches to Al-Bukamal. Another senior rebel source said they were now inside the town's administrative boundaries after a rapid advance across sparsely inhabited desert from their main base further southwest in al Tanf.

The rebels had advanced close to a railway station nearly five km from the outskirts of the Euphrates River town, approaching mine-laden trenches dug by IS as a line of fortified defense, two rebel sources said.

The militants also cut internet and electricity services in the town, which IS has controlled since 2014, they added.

U.S.-led coalition jets fired missiles at the town's Aisha hospital used by Islamic State but intense aerial bombing had not begun yet, Abdul Salam Muzil, a senior member of the New Syria Army in touch with commanders on the front, told Reuters.

But another New Syria Army commander said the rebels would be relying on heavier air strikes in coming days to help them encircle the town, believed to have around 50,000 people.

"This is a major gamble but we have been preparing for it for a long time. U.S. logistical support and aerial support is key to the success of this operation," said the commander, who like others requested anonymity.

In a video posted by the rebels, a commander is seen driving in a military vehicle through an undisclosed desert area and saying his troops were heading to "liberate the town".

The New Syria Army's base in al Tanf was hit twice earlier this month by Russian air strikes, even after the U.S. military used emergency channels to ask Moscow to stop after the first strike, according to U.S. officials.

Russia is Syrian President Bashar al-Assad's most powerful ally in his more than five-year-long war with rebels trying to overthrow him.

A U.S. Defence Department spokesman, Major Adrian J.T. Rankine-Galloway declined to comment on the latest campaign but said Washington was assisting unnamed Syrian rebel groups.

"Coalition-partnered forces will isolate, degrade, disrupt, destroy the Da'esh organization and seize their terrain and functional capabilities," the Pentagon spokesman said, using the Arabic acronym for Islamic State.

The U.S.-led campaign against Islamic State in Syria has gone up a gear this month, with an alliance of militias including the Kurdish YPG launching a major offensive against IS in the city of Manbij in northern Syria.

(Reporting by Tom Perry in Beirut, Suleiman al Khalidi in Amman and Yeganeh Torbati in Washington; Editing by Mark Heinrich)