UN Secretary General Kofi Annan says he won a pledge from Syria to increase border security with Lebanon and take steps to stop the flow of arms.
Mr Annan is in Syria ahead of a visit to Tehran
Mr Annan announced the move after talks with Syrian President Bashar al-Assad.
He said Mr Assad had given his full support to the UN resolution that ended the fighting between Israel and the Lebanese militant group Hezbollah.
But Mr Assad has expressed objections to Israel's call for a UN presence on the border between Syria and Lebanon.
He says any such move would be a hostile act. The foreign minister has threatened to cut all road links to Lebanon if that happens.
Mr Annan was in Damascus as part of a Middle East tour that on Saturday will take him to Iran, Syria's main regional ally which is also accused of backing Hezbollah.
During his hour-long meeting with Mr Assad, Mr Annan also asked for Syria's help in securing the release of two Israel soldiers whose capture by Hezbollah sparked Israel's month-long offensive.
In other developments:
- The Spanish government approved plans to send 1,100 troops to southern Lebanon
- UN humanitarian co-ordinator David Shearer said the humanitarian situation in Lebanon had been improving rapidly since the ceasefire began
- Italian Prime Minister Romano Prodi said Italy and France must use their growing relationship to tackle the Middle East crisis
Speaking to reporters at the airport before departing for Qatar, Mr Annan said Syria had agreed to increase patrols along the Syrian-Lebanese border and "when possible" to start joint patrols with Lebanese troops.
LEBANON AID PLEDGES
Total pledged - $940m
Largest donors include:
Qatar - $300m
United States - $234m
Arab Fund - $112m
Saudi Arabia - $60m
European Commission - $54m
United Arab Emirates - $50m
Italy - $38m
Spain - $34m
Mr Assad had agreed to take "all necessary measures" to implement paragraph 15 of UN resolution 1701, which deals with the arms embargo, he added.
He said he was confident the Syrian measures would work.
"I think it can happen," he said, quoted by the Associated Press news agency.
"It may not be 100%, but it will make quite a lot of difference if the government puts in place the measures the government has discussed with me. I have no reason to believe it will not be done."
The Syrian president has so far made no comment on the talks.
BBC diplomatic correspondent Jonathan Marcus says that while Hezbollah is more closely tied to Iran than to Syria, the Damascus government has given it important practical and political help.
Many of the missiles fired by Hezbollah against Israel were either Syrian-made or supplied via-Syrian territory from Iran, he says, and that is why Mr Annan has been eager to get Mr Assad's support for paragraph 15.
The clause says entities in Lebanon must not be sold or supplied weapons without the consent of the Lebanese government or UN peacekeepers.
However, our correspondent says that while diplomatic commitments are all very well, Israel and the US will be watching very closely to see what happens on the ground.
Mr Annan has already visited Israel, Lebanon and Jordan, in an attempt to strengthen the fragile ceasefire between Israel and Hezbollah, which came in effect on 14 August.
He has called on Israel to lift its air and sea blockade of Lebanon and withdraw all its troops from the south of the country as soon as 5,000 UN peacekeepers reach the area.