"The Rivers State government is working round the clock to get them released," Nigerian government spokesman Emmanuel Okah told news agency AFP. "Talks are going on between concerned parties on the issue and we are very hopeful."
He declined to give details on the talks, saying that some of the issues involved "have security implications which cannot be made public," he said.
Twenty-five people who were working on a site belonging to Shell in Nigeria's southern Rivers State went missing after they were attacked by about 70 armed men on Monday, Shell officials in Nigeria said.
Fourteen Nigerian soldiers were killed, according to military.
Shell in London said earlier that nine of the subcontrators had been released, while 16 others were still being held.
Responsibility for the attack was claimed by the Joint Revolutionary Council (JRC), a group of several separatist movements.
The JRC said in a statement published by several dailies that the attack was aimed at obtaining the immediate release of Alhaji Mujahid Dokubo Asari, the imprisoned leader of the Niger Delta People's Volunteer Force (NDPVF).
In October 2005 Asari was charged in an Abuja Federal High Court with treason, illegal assembly and conspiracy.
The separatist Movement for the Emancipation of the Niger Delta (MEND) issued a statement denying responsibility but saying "we salute the courage of all who stand and fight in the name of the liberation of the Niger Delta.
"Investors of the stolen wealth of the Niger Delta should brace up for terrible times which will come upon them very suddenly," MEND warned.
The Niger Delta is home to Nigeria's multi-billion-dollar oil and gas industry, but each of its inhabitants live on less than one dollar a day.
Nigeria, a nation of 130 million people, is Africa's leading oil producer and the world's sixth biggest crude exporter with a normal daily output of 2.6 million barrels.
But attacks on oil installations in the Delta region have cut about a quarter of oil production since the start of the year and at least five
Nigerian staff employed in the oil sector have been killed by unknown attackers.
Since January, more than 40 expatriate oil workers have been kidnapped but later released, while 25 soldiers or security officers have been killed.
Following an increase in the number of abductions in August, the Nigerian army launched a widespread manhunt for the kidnap gangs on the orders of President Olusegun Obasanjo.