Libya's regular border security guards have stopped working at the main border crossing to Egypt, officials said on Wednesday, a further breakdown of state authority.
The incident will reinforce worries in Egypt that militants are using the desert border to smuggle in fighters and weapons from Libya, which is in a state of chaos four years after rebels overthrew Muammar Gaddafi.
The border is also a transit route for migrants from Syria heading via Egypt to western Libya, from where they hope to get by boat with the help of smugglers to Italy.
"The force protecting the Musaid border crossing between Egypt and Libya have withdrawn due to some problems," said Tariq Kharaz, a spokesman for the interior ministry of Libya's internationally recognised government, based in the east.
He said another force was now manning the border, declining to say why the regular troops had left.
An Egyptian security source said the regular Libyan border guards had stopped working five days ago, adding that civilians now appeared to be working on the Libyan side of the border.
Egypt had increased before security at the 1,000 km (600 mile) long border with Libya, where Islamic State has made inroads by exploiting a security vacuum as two governments fight each other.
The internationally recognised government is based in the east since a rival group seized the capital Tripoli a year ago, setting up its own administration.
Both administrations rely on alliances of former anti-Gaddafi rebels which lack equipment and training.