The US embassy in Indonesia issued a security alert on Saturday for its citizens in the country’s second-biggest city, Surabaya.

“The US embassy has been made aware of a potential threat against US-associated hotels and banks in Surabaya, Indonesia,” the embassy said in a statement on its website.

“The US embassy recommends heightened vigilance and awareness of one’s surroundings when visiting such facilities,” it said.

The statement gave no further details.

Indonesia is considered to have been largely successful in cracking down on homegrown militant groups following multiple attacks in the 2000s targeting foreigners and state institutions.

But authorities have raised concerns in recent months over hundreds of Indonesian citizens attempting to travel to Syria and Iraq to fight alongside militants of the Islamic State (Isis) organisation.