UN aid chief Valerie Amos has said the Syrian government has agreed to a "limited assessment" of the humanitarian situation in the country.
At a news conference in Turkey, Baroness Amos said she had asked for full access to the worst-hit areas, but the government had asked for more time.
She has been touring camps on the Turkish-Syrian border where about 11,000 Syrians have taken refuge.
Earlier, opposition activists rejected calls for dialogue by envoy Kofi Annan.
Burhan Ghalioun, the leader of the main opposition, said the comments were "disappointing" when Syrians were being "massacred every day".
Mr Annan, who is due in Syria on Saturday in his role as Arab League and UN envoy, had said a solution lies in a "political settlement".
He also said further militarisation "would make things worse".
Mr Annan is scheduled to meet Syrian President Bashar al-Assad at the weekend.
Meanwhile, Turkish media reports said four Syrian brigadier-generals had defected and fled into Turkey.
Baroness Amos, who visited the city of Homs including the bombed-out district of Baba Amr this week, said it was vital to get unhindered humanitarian access to areas that needed it.
"The government have agreed to a limited assessment exercise to be conducted by UN agencies and the Syrian authorities which would give us some information about what is happening in the country," she said.
"We continue to need a more robust engagement that would enable us to have more information about what is happening."
But she said the Syrian government had asked for more time to look at the agreement that she put to them.
The UN aid chief said she was "horrified" by the destruction she saw in Baba Amr, which was vacated by rebels last weeks after a massive government bombardment lasting several weeks.
"Almost all the buildings had been destroyed and there were hardly any people left there," she said.
"I am extremely concerned as to the whereabouts of the people who have been displaced from Baba Amr."
Tens of thousands of people were again reported to be on the streets across Syria on Friday, protesting against the Assad regime.
Activists from the UK-based Syrian Observatory for Human Rights said there were big demonstrations in Deraa, Latakia, Homs, Hama, Deir Ezzor and Aleppo.
The Local Co-ordination Committees, another rights group, said 46 people were killed across Syria on Friday, including 22 in Homs, 11 in Idlib, five in Deraa, four in Hama, two in Damascus and one each in Latakia and Aleppo.
Meanwhile, activists and the Observatory said troops backed by tanks were massing in Idlib, near the Turkish border, to target the Free Syrian Army (FSA), the country's main rebel military force.
Some activists fear Idlib could suffer the same fate as Baba Amr.
The UN says more than 7,500 people have died as a result of the violence in Syria over the past 12 months.
In other diplomatic moves on Friday, Russia said it could not back a new UN draft resolution on Syria as it was "unbalanced".
Deputy Foreign Minister Gennady Gatilov told Interfax: "Its main problem is the absence of a simultaneous call on all sides to take practical steps in the context of ceasing fire."
The draft resolution demands the Syrian government "immediately" ends violence while calling on opposition groups to "refrain from all violence".
Russia and China have jointly vetoed two previous UN Security Council drafts.