United Nations staff say the Syrian army has stopped them from reaching the village of Qubair, where 78 people are reported to have been killed.
"They are being stopped at Syrian army checkpoints and in some cases turned back," the UN mission commander said.
The head of the UN also claimed they had been shot at.
United Nations monitors, who are not armed, are in Syria to check if the peace plan is working there.
UN staff are are normally treated as 'neutral' and are allowed to move around the country.
In Syria a peace plan was set up by Kofi Annan who represents the UN and a number of other Middle Eastern countries.
On Thursday he, along with other world leaders, stepped up calls for Syria to accept the plan.
The plan was supposed to bring in a ceasefire in mid-April, but the country has continued to suffer from violent attacks.
Chaos and confusion
The most recent attack on Thursday morning, in the village of Qubair near the city of Hama, is said to have killed 78 people.
Many of the victims are said to be women and children.
Lots of people claim the Syrian government is behind the attack - but the President, Bashar al-Assad says that it was terrorists.
It's difficult to know exactly what is happening because chaos and confusion are increasing in the country.
The United Nations is still trying to help work out a solution but things aren't getting any better.
No one has been able to agree on what to do next but talks are continuing to try and work out what the rest of the world, including the UK, could do to help.