Foreign Secretary William Hague warned that Syria was at a "fork in the road" tonight, amid widespread condemnation of the regime's bloody repression of protests.
Mr Hague said president Bashar Assad had to commit to "radical reform" that would bring long-term peace and stability, or face international sanctions.
The intervention came as UK diplomats were trying to negotiate a joint statement with United Nations (UN) partners condemning violence in the troubled Middle East state.
Thousands of soldiers backed by tanks were yesterday reported to have poured into the city of Daraa, where the uprising against British-educated Assad began, before dawn.
They reportedly opened fire indiscriminately on civilians, and tanks later moved in as electricity, water and mobile phone services were cut.
Knife-wielding security agents made house-to-house sweeps in what activists called a campaign to intimidate protesters.
Other crackdowns and arrest sweeps were reported on the outskirts of Damascus and the coastal town of Jableh, bringing more international condemnation.
Human rights groups estimate that around 350 people have died since the start of the unrest in March.
As the violence escalated, the White House said it was considering sanctions against the Syrian government - the first official indication that such a move was being contemplated.
Updating MPs on the situation today, Mr Hague said: "The United Kingdom is working intensively with our international partners to persuade the Syrian authorities to stop the violence and respect basic and universal human rights to freedoms of expression and assembly.
"Syria is now at a fork in the road. Its Government can still choose to bring about the radical reform which alone can provide peace and stability in Syria and for the long term, and we urge it do so.
"Or it can choose ever more violent repression, which can only bring short term security for the authorities there.
"If it does so we will work with our European partners and others to take measures, including sanctions, that will have an impact on the regime."
Syrian human rights groups said as many as 500 pro-democracy sympathisers have been detained across the country, many in Damascus and Jableh, while phone lines with Daraa have been cut.
The US State Department ordered non-essential embassy staff and the families of all personnel to leave the country.
The Foreign Office is advising against all travel to Syria, and has urged British nationals with no pressing need to remain to leave now by commercial means.Reuse content