SINGAPORE (AFP) -- Islamic State jihadists are eyeing expansion into Southeast Asia by joining forces with local extremists, a senior US counter-terrorism official warned Friday.

IS has a history of partnering with militant groups around the world, including in Egypt, Libya and Nigeria, and wants to broaden its reach in the region, according to Justin Siberell, acting coordinator for counter-terrorism at the US State Department.

"My understanding is that they have looked at existing groups across the region," Siberell said in a conference call from Washington with Asia-based journalists.

"There have been people that have pledged affiliation and allegiance to IS at the group level. We're certainly concerned about that, we're concerned about the rise of new IS affiliates and we're working with governments to do what they can to prevent that."

Siberell also noted that militants from Southeast Asia fighting with IS in Iraq and Syria have been deployed in a unit called the Katibah Nusantara, and could pose a threat when they eventually return to their home countries.

"We're certainly concerned about IS' ability to expand or to establish branches," he said.

There have been only relatively minor attacks and plots blamed on IS affiliates in the region, but analysts fear the group could become more effective.

Indonesian police earlier this month arrested six suspected militants over a plot to launch a rocket attack on an up-market Singapore waterfront district from the nearby Indonesian island of Batam.

The suspects' alleged leader, Gigih Rahmat Dewa, is accused of planning the attack with Bahrun Naim, a leading Indonesian militant who is believed to be fighting with IS in Syria.

In January IS-linked militants launched a deadly gun and bomb attack in Jakarta which left four attackers and four civilians dead.

Singapore on August 19 announced it had detained two men under its tough internal security law after discovering they intended to travel to Syria to fight for IS.

Siberell spoke Friday after he travelled to Bali earlier this month for a meeting on preventing the cross-border movements of known or suspected terrorists. He also visited Jakarta, Malaysia and Singapore before returning to Washington.