Nigeria's Department of Petroleum Resources has invited workers of the senior staff union, Pengassan, for a meeting over complaints of pay and conditions.

Pengassan staff began a strike on Monday in support of demands including greater autonomy, higher pay, and payment of allowances arrears, some dating back to 2000.

Blue-collar staff at the DPR on Tuesday joined the strike, which has threatened Nigeria's oil output of more than 2 million barrels a day.

Pengassan members monitor the quantity and quality of crude leaving Nigerian terminals, and some observers have said the strike threatens to cripple the country's export capability.

A long-running strike could have severe repercussions for the Nigerian economy, as oil exports account for more than 80% of government revenue and more than 90% of foreign exchange earnings.

There were reports of long queues of cars at petrol stations in Lagos, the country's commercial capital, and the capital Abuja, as panic buying of petrol began.