Iran's biggest crude oil tanker operator NITC will need to review whether to send its vessels to Libya if it receives a new order, a senior NITC official has said.

The move threatens to further reduce sailings from Africa's third-largest producer, whose shipments have been hit by falling oil output, the impact of sanctions and rising freight costs.

An NITC tanker left the eastern Libyan port of Tobruk on Tuesday with around one million barrels of crude oil bound for Europe, the official said.

'We have no programme to send more,' the official said.

'There has not been any demand for Libya. If there was any demand we will have to think about it.'

The official said it would need to consult its legal department on any fresh sailings.

Shipping sources said earlier major Libyan oil ports Ras Lanuf and Brega in the east of the country were closed as violence in the area had hampered operations at the terminals.

The NITC official said it had no difficulties loading at Tobruk.

'The port was very quiet, some other oil ports are not operational,' the official said.

Before the turmoil, Libya pumped around 1.6 million barrels per day (bpd).

The International Energy Agency said last week 1 million bpd of oil production was shut after foreign oil firms evacuated their workers.