DUBAI, UNITED ARAB EMIRATES, JAN. 15 -- Iranian gunboats attacked a Liberian tanker in the southern Persian Gulf today hours after ambushing a Norwegian tanker at the entrance to the waterway, shipping sources said.

They said a U.S. warship contacted the attacked vessel, a 36,750-ton Liberian ship, after it sent out an emergency call.

It reported being hit 15 miles off the United Arab Emirates port of Jebel Ali.

The attack was the third by Iran in two days. It followed two unconfirmed reports by Iraq that its planes had hit two Iranian ships.

Iranian gunboats earlier today raked a 13,524-ton Norwegian liquid petroleum gas carrier with machine-gun and rocket-propelled-grenade fire as it sailed fully loaded out of the Persian Gulf from the Saudi Arabian port of Jubail. Two crewmen were slightly injured.

As the tanker war intensified, Tehran's top war spokesman said Iranian ground forces along the front line with Iraq were on full alert awaiting the order to start a new offensive.

"We are now in the best position for restoring our rights," Tehran Radio quoted Hojatoleslam Ali Akbar Hashemi Rafsanjani, Iran's speaker of parliament, as saying at Friday prayers in Tehran.

A 10-day lull at the start of January had increased hopes that Syrian-broked efforts to mediate between Iran and Iraq would succeed, but the wave of shipping attacks resumed last Sunday after Iraqi jets attacked an Iranian supertanker.

"The only way to stop the tanker war would be to prevent Iraq from attacking Iranian shipping," a shipping source said.