By Alex Last
BBC News, Lagos
A three-day strike by Nigerian oil workers has had some impact on production whilst queues are being reported at petrol stations.
Oil militants have caused a 25% drop in oil output
Union officials say thousands of oil workers have obeyed the order to stay away from work protesting at insecurity in the oil-producing Niger Delta.
They want the government to do more to protect workers in the region from attacks and abductions by armed groups.
On Tuesday a Nigerian oil worker was killed in an attack on a supply ship.
Reports suggest the unions and government are holding more meetings to try and end the action.
Nigeria is the world's eighth largest producer of crude oil.
A shipping agent told Reuters news agency that tanker loadings were suspended at two major terminals, Brass River and Escravos.
But industry officials said operations appeared to continue normally at other ports.
"We are working to maintain production at normal levels," said Exxon spokeswoman Gloria Essien.
The unions told thousands of their workers not to work for three days as a warning to the government to do more to stop attacks on the oil industry by armed groups.
This year there have been a string of raids on oil installations and kidnappings of oil workers.
On Tuesday a Nigerian oil worker was killed in an attack by pirates on a ship operated by an American oil services company. Two others were injured, Chevron announced..
Last month, an employee for Shell who had been held hostage was killed in a shoot-out between troops and militants.
The unions say their strike is targeted against the government rather than the oil companies working in the Delta.
There have been talks among the parties about limiting the strike's impact on oil production.
The BBC's Alex Last in Nigeria says that in past stoppages, the oil companies have used a few key staff and non-unionised workers to keep the oil flowing but a prolonged strike could have consequences.
Nigeria is Africa's largest oil producer but its output is already down by more than a quarter as a result of militant attacks and pipeline leaks.
This has contributed to the high global price of crude oil and despite government pledges to take a tough line with the armed groups there are fears of continued insecurity in the oil producing region in the run-up to Nigeria's elections next year, he says.