WASHINGTON - The nation's top military officer said Tuesday the United States did all it could to intercept a suspected arms shipment to Hamas militants in the Gaza Strip, but its hands were tied.

Separately, Admiral Mike Mullen, the chairman of the Joint Chiefs of Staff, and other U.S. officials said it's too soon to tell whether the prospect of new U.S. engagement with Iran will bear fruit.

Mullen confirmed that a Cypriot-flagged ship intercepted in the Red Sea last week was carrying Iranian arms.

He says U.S. authorities suspect the shipment was ultimately bound for the Gaza Strip, where Hamas and Israel are observing a shaky truce after three weeks of fighting.

Mullen says in spite of its suspicions, it lacked the authority to seize the suspect vessel.

Military officials say the U.S. Navy searched the ship with permission of the captain and found small munitions.

The vessel was allowed to continue its voyage after the search, and Mullen said the ship was expected in port in Syria this week.

Israel launched a 22-day offensive late last month on Hamas-controlled Gaza to try to permanently halt years of militant rocket fire on growing numbers of Israelis.

It also wanted to halt the smuggling of arms that turned Hamas into a threat to much of southern Israel.

Mullen said he favours approaches to Iran that would be a strategic benefit to the United States. He says stability in Afghanistan is in both nations' interest.

"To the degree that we are able to dialogue with them, find some mutual interests, there is potential there for moving ahead together."

He offered a heavy dose of caution.

"Iran is unhelpful in many, many ways in many, many areas, and so I wouldn't be overly optimistic at this point," Mullen told reporters at the Foreign Press Centre.