FALLUJAH, Iraq, Jan. 7 (Xinhuanet) -- Fierce clashes broke out between insurgents and US forces in the flashpoint city of Fallujah on Saturday, witnesses said.

"A sniper opened fire at a group of US soldiers, shooting four of them in al-Wahda district in central Fallujah," a local journalist told Xinhua on condition of anonymity.

Meanwhile, armed men battled with the US troops in al-Tharthar Street in the eastern part of the city as the latter tightened security measures, blocking all main entrances to the city, he added.

Earlier in the day, a roadside bomb went off near a US military patrol at about 7:30 a.m. (0430 GMT) in eastern Fallujah as a US military patrol was passing by, destroying a US Humvee, killing or wounding the soldiers aboard, the source said.

In a separate incident, local residents reported fierce clashes between US soldiers and militants in the Arba'ien Street incentral Fallujah.

They said they didn't know the exact number of the casualties as the clash was heavy and both sides used different kinds of weapons.

The Sunni-dominated Fallujah, some 50 km west of Baghdad, has witnessed sporadic attacks against the U.S. and Iraqi government forces despite U.S.-Iraqi military operations in the city to stamp out insurgency.