ASMARA -- The United States has warned US citizens against visiting Eritrea after authorities in the often-secretive Horn of Africa nation slapped new in-country travel restrictions on foreigners.

The State Department advised Americans to defer all non-essential travel to Eritrea, citing the new rules that were imposed amid heightened border tensions with its archrival neighbor Ethiopia.

"Due to these revised travel restrictions, the US embassy cannot guarantee ... assistance in an emergency outside of Asmara and the Department of State recommends US citizens defer all non-essential travel at this time," it said.

"The [government] has cited security and safety concerns as the reasons for requiring travel permits for all travel outside Asmara," the department said in a statement released in Washington late on Monday.

It noted continuing tensions along with Eritrea-Ethiopia border, and also that several international aid organizations had been expelled from the country in the last year on short notice with little or no explanation.

Asmara imposed the new restrictions on June 1, requiring all foreigners, including resident diplomats and aid workers, as well as temporary visitors and tourists, to have permits to travel within the country.

No official explanation was given at the time and Asmara-based diplomats said that they did not understand the reasoning for the rules issued as ties between Eritrea and the international community become increasingly strained.

The permits, which generally take several days to be processed, require authorities to approve specific dates and times of travel, the route to be taken and the reason for the trip.

Eritrea is currently under heavy pressure to remove restrictions that it imposed last year on UN peacekeepers monitoring its tense border with Ethiopia.