Continued shelling in the Syrian city of Homs is stopping Red Cross teams from evacuating scores of civilians.
Government forces and rebel fighters agreed on Wednesday to a two-hour ceasefire to allow aid workers into the worst affected areas.
However, correspondents say it may be hours or days before they can enter.
Meanwhile, a Syrian Mig 21 fighter jet made an emergency landing in neighbouring Jordan, at a base in the north of the country, reports said.
Syrian state TV earlier announced that contact had been lost with a fighter plane, being flown by an air force colonel, during a training mission.
A Jordanian security source told AFP that the plane had landed at the King Hussein air base in Mafraq.
Syrian activists quoted by Reuters said the pilot had defected, but this has not been confirmed.
In other developments, reports in the British media say the UK and US could grant Syrian President Bashar al-Assad clemency if he joins peace talks.
The move was discussed on the sidelines of the G20 summit on Mexico, the Guardian newspaper reported.
On Wednesday, the International Committee of the Red Cross (ICRC) said that its teams, along with the Syrian Arab Red Crescent, were poised to evacuate civilians and deliver aid into parts of Homs.
Waleed Faris, a resident of one area the ICRC was trying to enter, said shelling had been heaviest at dawn on Thursday but there were signs it was subsiding.
"Early this morning there was heavy shelling. Now I can hear one or two mortars fall every half an hour. It is quiet today compared to the past few days," he said, quoted by Reuters.
The ICRC's regional head of operations, Beatrice Megevand-Roggo, said fighting had been raging for more than 10 days and hundreds of people were trapped.
Heavy shelling by government forces in residential areas of the city has killed hundreds of civilians and brought international condemnation.
The Syrian government blames the deaths on "terrorists" backed by foreign powers.
Water and electricity to many areas have been cut for some time and living conditions are reported to be dire.
The BBC's Jim Muir in Beirut says it appears some practical details could also be holding up the mission, such as how many aid vehicles are allowed in and where the wounded will be transferred to.
It could take hours, or days, to resolve, he says.
Meanwhile, diplomatic efforts to end the crisis are continuing.
UK officials quoted by the Guardian said Britain was willing to discuss granting clemency to President Assad if the move would allow a UN-sponsored conference in Geneva on political transition to be launched.
UK Prime Minister David Cameron and US President Barack Obama are said to have received encouragement from Russian President Vladimir Putin in talks at the G20 in Mexico.
Correspondents say a deal could protect Mr Assad from prosecution and also allow him safe passage to another country, possibly Iran or Russia.
Russia, a close ally of Damascus, is a major supplier of arms to Syria, but insists that its shipments do not violate international laws.
In an interview with the Russian Interfax news agency on Thursday, Arab League deputy secretary general Ahmed Bin Hilli called on Moscow to stop providing military supplies to Syria.
"Any assistance to violence must be ceased because when you supply military equipment, you help kill people. This must stop," he is reported to have said.
Russia and China have twice blocked UN resolutions condemning the Syrian regime for the violence.
They argue that pushing a government from power using external pressure is unacceptable.
Earlier this week, UK officials said a Russian ship believed to be carrying military helicopters to Syria had turned back after its British insurer removed coverage for the vessel.
The MV Alaed was last seen off the north-west of Scotland.
The ship's owners, Russia's Femco cargo line, later issued a statement denying the vessel was transporting anything illegal.
Last week, US Secretary of State Hillary Clinton accused Moscow of supplying Syria with attack helicopters.
President Putin's spokesman denied this, saying Russia was only maintaining helicopters sold to Damascus "a very long time ago".