The United States has warned Americans against travel to the southern Philippines just days after Australia and Canada issued similar advice.
A "security message" from the US embassy in Manila earlier this week said diplomatic staff had been restricted from travelling to three cities on the southern island of Mindanao.
It was followed by a US State Department travel warning dated July 5 advising Americans to exercise extreme caution when travelling to Mindanao, citing "criminal gangs... (and) terrorist groups" in the area.
It also called on Americans to defer non-essential travel to the Sulu archipelago, a chain of islands off Mindanao where Al-Qaeda-linked militants are active.
"It (the travel warning) reflects continuing threats in the Sulu archipelago and the island of Mindanao due to terrorist and insurgent activities," the US embassy said in a statement.
Australia and Canada issued warnings on Wednesday about fresh threats of terrorism and kidnapping in the southern Philippines.
Canberra has barred its diplomats from travelling to Davao, Cotabato and Zamboanga -- the same three Mindanao cities that US embassy staff are not allowed to travel to.
An embassy spokeswoman declined to comment on the nature of the threat against Americans.
The Philippine national police also did not know of any specific threat against Americans in Mindanao, said spokesman Senior Superintendent Reuben Sindac.
Mindanao and surrounding islands are a hotbed of various armed groups including communist guerrillas, bandits, Muslim insurgents and the Abu Sayyaf, an Islamic extremist group blamed for the country's worst terror attacks.
Founded using seed money from Al-Qaeda mastermind Osama bin Laden in the 1990s, the Abu Sayyaf has often targeted foreigners for kidnappings for ransom.
In 2001, the group abducted three American tourists among a group of hostages from a Philippine resort, leading to the deaths of two of the Americans.
US troops have been based in the southern Philippines for more than a decade to help train local troops in hunting down members of the Abu Sayyaf, which is on the US government's list of so-called foreign terrorist organisations.