Britain's foreign secretary has demanded "indisputable signs of change" from the Syrian regime as the UN prepares to send in monitors.
Security council members will debate a draft resolution sending in up to 30 officials later on Friday, after Bashar al-Assad's military began a fragile ceasefire at dawn on Thursday.
The tentative halt to hostilities is set to be severely tested later as pro-democracy campaigners demand demonstrations after Friday prayers.
Reports of shooting and explosions in a Damascus district, separate reports of a roadside bomb killing an officer in a troop transporter and the deaths of three people by snipers are all raising concerns.
"The Syrian government has a record of failing to keep its promises. It has the opportunity to change that now," William Hague said yesterday.
"We need to see visible, verifiable and indisputable signs of change. The opposition must also ensure that they adhere to the ceasefire and work to strengthen and broaden it."
Up to 10,000 people are believed to have died as a result of the one-sided fighting in Syria which emerged from 2011's Arab Spring.
Former UN secretary-general Kofi Annan's six-point plan, which includes requirements on the authorities to respect freedom of movement for journalists as well as the right to demonstrate peacefully, is slowly being implemented.
But the Syrian government has not yet withdrawn its armed forces from towns and cities, meaning the deal has not yet been fully implemented.
Mr Hague said the British government would keep up its pressure on the Syrian government on the six-point plan, "including the move towards a political transition which will bring the Syrian people the greater freedoms they deserve".
The mood in Syria remains one of deep uncertainty.
"I hope that the government will respect the ceasefire. This is an international affair. But will the gunmen abide?" one Damascus resident, Nidal, told the AFP news agency.
Another said: "I am telling you, Syria has become a base. Russia and China would not let it fall."