Libya's National Oil Corp (NOC) has accused the United Arab Emirates of instructing eastern forces in Libya's civil war to reimpose a blockade of oil exports after the departure of the first tanker in six months.

The UAE, along with Russia and Egypt, supports the eastern-based self-styled Libyan National Army (LNA) of renegade military commander Khalifa Haftar, which on Saturday said the blockade would continue despite it having let a tanker loaded with oil from storage.

Libya, which sits atop Africa's largest proven crude oil reserves, is torn between the rival powers of the UN-recognised Government of National Accord (GNA) in Tripoli and eastern-based Haftar.

"NOC has been informed that the instructions to shut down production were given to (the LNA) by the United Arab Emirates," it said in a statement on Sunday, resuming force majeure on all oil exports.

There was no immediate comment on NOC's accusation from either the LNA or the UAE.

Haftar has been on the back foot after Turkish support helped the GNA turn back his 14-month assault on the capital, Tripoli.

On Friday the Vitol tanker Kriti Bastion docked and loaded at Es Sider port before sailing on Saturday, the first legal export of Libyan oil since the blockade was imposed in January.

The NOC said Russian and Syrian mercenaries fighting alongside the LNA now occupied Es Sider.

Under international agreements, only NOC can produce and export oil and revenues must flow into the Central Bank of Libya. Both those institutions are based in Tripoli, the seat of the GNA.

The LNA said on Saturday it would continue the blockade until a list of conditions were met, including channelling oil revenue into a new bank account based outside the country to then be distributed regionally.

On Sunday the United States's Libya embassy said the resumption of the blockade came after "days of intense diplomatic activity" to let NOC resume output, and said it "regrets that foreign-backed efforts" had impeded this.