The deadline for the release of 10 hostages who have been held by Somali pirates for six weeks on a UN-chartered ship has passed.
The ship is carrying food for Somali victims of the tsunami
Under a deal agreed with the hijackers on Friday, the ship was due to dock at the port of El Maan near the capital, Mogadishu, within three days.
But the fate of the MV Semlow, which was taking aid to Somali victims of the December tsunami, remains unclear.
UN officials say the pirates have not been in touch since the agreement.
The officials working for the UN's World Food Programme (WFP) in Mogadishu say they do not know whether contact has broken down or not.
Abeyle Salah, a worker at the port, said: "They have been waiting for the ship since last night, but it has not come."
The WFP said at the weekend that a deal had been reached on Friday for the safe return of the ship, its crew and the 850 tons of rice it is carrying within days, on the condition that the rice would be distributed in central Somalia, where the pirates come off.
Osman Abdi, one of the elders involved in the negotiations, said at the time that the pirates initially demanded a $500,000 ransom for the release.
They later reduced it, demanding only the rice donated by Japan and Germany for tsunami victims in the north-eastern city of Bosaaso, he said.
It is still unclear what changed the mind of the pirates, but sources in the region say there is a dispute among themselves over the agreement, says the BBC's Mohammed Olad Hassan in Mogadishu.
The Semlow was seized at the end of June in seas between the areas of Haradheere and Hobyo, north-east of Mogadishu, as it sailed from the Kenyan port of Mombasa.
The crew comprises eight Kenyans, a Tanzanian and their Sri Lankan captain.
WFP provides an average of 3,000 tons of aid a month to 275,000 people in Somalia.
But its work is hampered by the fact that the country is awash with some 60,000 militiamen.
Somalia has had no functioning national government since 1991.
Attempts to relocate a new transitional administration - set up in neighbouring Kenya last year - back in Somalia have so far failed.
The waters off the Somali coast are among the most dangerous in the world, the International Maritime Board says.