EU countries are discussing imposing sanctions on Syria if the regime continues its violent suppression of pro-democracy protests.
The foreign secretary, William Hague, said Damascus faced "a fork in the road" as opposition activists reported continuing government attacks in the southern town of Deraa and mass arrests and tanks in areas including Douma near the capital and Baniyas on the coast.
In Rome, Nicolas Sarkozy and Silvio Berlusconi issued a joint call for an end to violence against the demonstrators. The French president described the situation as "unacceptable" but also made it clear that he was not contemplating direct intervention of the sort he championed against Muammar Gaddafi in Libya.
"We are not going to intervene everywhere in the world and not all situations are necessarily the same," Sarkozy said.
Possible EU sanctions would probably include travel bans and asset freezes targeting members of President Bashar al-Assad's family and other senior regime figures, and would be co-ordinated with US punitive measures being prepared by the Obama administration.
"Syria is now at a fork in the road," Hague told MPs. "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 to 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."
But the difficulties of concerted international action were illustrated when members of the UN security council debated a statement criticising Syria as Russian diplomats sought to water down a European draft. The statement would not have the legal force or direct impact of earlier resolutions against Libya. Diplomats said the Syrian government had more defenders on the international stage, and – having endured years of US sanctions already – was less vulnerable to new pressure.
The Syrian human rights organisation Sawasiah said at least 400 civilians had been killed during a month of protests including at least 35 in Deraa since Monday. It said 500 people had been arrested in the past few days after the abrogation of the country's decades-old emergency law, one of several concessions by Assad widely dismissed as too little and too late.
Despite an attempted media blackout, reports from Syria described more troops, snipers and armoured vehicles in Deraa, where the uprising began last month.
Al-Jazeera Arabic quoted residents as saying shelling had killed 20 victims, including women and children. Water, electricity and phone lines had been cut but information was filtering out via locals using Jordanian mobile phones. Amnesty International said at least 23 people had been killed by tank fire. "Sometimes you hear a burst of heavy machine-gun fire coming in all directions as though to just scare people," one resident told Reuters.
The central city of Homs was also described by opposition sources as "under siege", as was the northern town of Baniyas, where Sunnis live in the heart of an area dominated by the minority Alawite sect, to which the Assad family belong.
Violence peaked at the weekend with 150 people killed in just three days.
Opposition supporters abroad said on Tuesday that there were now fewer videos emerging of the violence, apparently reflecting the arrest of activists and a more effective security crackdown. Arab media also highlighted claims of defections from the Syrian army, but it was hard to assess the scale and significance of this.
Views of the situation have hardened since last Friday's violence, and the storming of Deraa on Monday showed Assad had decided to use brute force, not further reforms, to deal with the protests.
But for all the talk of sanctions, the emphasis in Europe was still on offering Damascus an opportunity to change its behaviour. Hague said he had been in contact with the Turkish government over Syria, most recently on Monday night with its foreign minister, Ahmet Davutoglu. The Foreign Office sees Turkey as the most important intermediary in contacts with Damascus, a role that Ankara has sought to cultivate over recent years. The Turkish prime minister, Recep Tayyip Erdogan, has telephoned Assad to urge "restraint", while Turkey's ambassador to Syria expressed his country's "deep concern and sorrow over the loss of many lives" in a meeting with the Syrian prime minister, Adel Safar.