Two boats brought about 20 marines to the fishing village of Maydh in the northwestern enclave of Somaliland on Tuesday, where they showed pictures of suspected "terrorists" to locals before leaving after a few hours, residents said.
"They met some of the fishermen and the people and they showed some pictures they were carrying, saying that these people are terrorists that they are trying to capture," Assistant District Commissioner Ali Abdi said.
It was not clear who the marines were looking for. The United States set up a Horn of Africa counter-terrorism force in 2002.
Somalia's privately-owned Jamhuriya and Ogaal newspapers and Somaliland Television also carried reports of the marines' visit, quoting their own correspondents in the area who had interviewed fishermen on the coast using radio sets.
Armed gangs have carved up
Somalia into virtual fiefdoms
US officers at the Pentagon in Washington denied that any US forces had "landed" in Somaliland, although they did not volunteer an explanation as to how the various Somali sources that carried the reports could have been mistaken.
"The report is false. The marines have not landed," Marine Corps Lieutenant-General James Conway, director of operations for the US military Joint Chiefs of Staff, told reporters at a Pentagon briefing on Thursday.
"There is no indication that the people in the Joint Task Force Horn of Africa have landed on the Somalia coast. They're still in Djibouti and conducting operations," he said, referring to the US operation set up in 2002.
The reports of the marines' arrival coincided with witness accounts of US military activity elsewhere along the coast of Somaliland, a relatively stable region which broke away in 1991 to escape chaos engulfing the rest of Somalia. It is not internationally recognised.
"The report is false. The marines have not landed"
director of operations,
US Joint Chiefs of Staff
Two US military helicopters flew low over parts of the Gulf of Aden port of Berbera on Wednesday, including the docks, airport, a fuel depot and former barracks, residents said.
Reporters said three US vessels, including a helicopter carrier, had been spotted on Tuesday at the port of Las Qorei, where marines questioned fisherman about local shipping.
Somaliland's interior minister declined to comment.
Washington fears al-Qaida cells may be seeking new havens in the Horn of Africa where weak political institutions and poor policing of deserts and coastlines might provide places for armed men to plan attacks on Western targets elsewhere.