BEIRUT—The European Union will impose harsher sanctions on Syria, a senior EU official has said, as Russia tried to broker talks between the vice president and the opposition to calm violence.

Activists reported at least 50 killed in the regime’s siege of the restive city of Homs, which has become the epicenter of the worst violence of the 11-month-old uprising, which appears to be morphing into a civil war with fearsome sectarian overtones.

Syria’s third-largest city has become the major center of both resistance and reprisal, fueled in part by increasingly bold army defectors who want to bring down President Bashar Assad’s autocratic regime by force. Every day, rockets and mortars fired by regime forces rattle the streets of Homs. Armed rebels ambush government military checkpoints. Hatreds brew on either side of the avenues that divide the bloodstained Syrian city.

In Brussels, a senior EU official said the 27-nation bloc would soon impose harsher sanctions against Syria as it seeks to weaken Assad’s regime.

The official said the new measures might include bans on the import of Syrian phosphates, on commercial flights between Syria and Europe, and on financial transactions with the country’s central bank. The European Union imports 40 percent of Syria’s phosphate exports.

The official, who spoke on condition of anonymity in keeping with EU rules, said some measures would be adopted at the EU foreign ministers meeting on February 27. But he stressed the nature of the measures to be adopted remained unclear since the ministers are concerned over the impact on the Syrian public.

Russia, a close ally of Syria, and the West are pushing down starkly different paths in trying to deal with Syria’s nearly 11 months of bloodshed.

After blocking a Western and Arab attempt to bring UN pressure on Assad to step down, Russia has launched a bid to show it can resolve the turmoil.

Moscow is calling for a combination of reforms by the regime and negotiations, without calling for Assad to go. Its provisions are so far finding no traction with the opposition, which dismisses promises of reform as empty gestures, refuses any negotiations while violence continues and says Assad’s removal is the only option in the crisis.

Russian Prime Minister Vladimir Putin said outside forces should let Syrians settle their conflict “independently.”

“We should not act like a bull in a china shop,” Putin said on Wednesday, according to the Itar-TASS news agency. “We have to give people a chance to make decisions about their destiny independently, to help, to give advice, to put limits somewhere so that the opposing sides would not have a chance to use arms, but not to interfere.”

Russian Foreign Minister Sergey Lavrov, who met with Assad on Tuesday in Damascus, told reporters in Moscow that the Syrian president delegated to his vice president, Farouk al-Sharaa, responsibility for holding a dialogue with the opposition.

Lavrov blamed both Assad’s regime and opposition forces for instigating the violence, which the UN says has killed well over 5,400 people.

“On both sides, there are people that aim at an armed confrontation, not a dialogue,” Lavrov said.

Rebel soldiers are playing a bigger role in Syria’s Arab-Spring inspired uprising, turning it into a more militarized conflict and hurtling the country ever more quickly toward a civil war.

In their meeting on Tuesday, Assad said the government was ready to talk to the opposition and would cooperate with “any effort that boosts stability in Syria.”

The regime’s crackdown on dissent has left it almost completely isolated internationally and facing growing sanctions. The US closed its embassy in Damascus on Monday and five European countries and six Arab Gulf nations have pulled their ambassadors out of Damascus over the past two days. Germany, whose envoy left Syria this month, said he would not be replaced.

The UN’s top human rights official Navi Pillay called on nations to immediately act to stop the bloodshed, saying she was “appalled” by the Syrian regime’s offensive against the central city of Homs, where activists say hundreds have been killed since Saturday. She said the killings show an “extreme urgency for the international community to cut through the politics and take effective action to protect the Syrian population.”

In New York, UN Secretary General Ban Ki-moon told reporters that the Arab League planned to send observers back to Syria and had asked the UN to consider a joint mission.

The UN chief provided no specifics, but the idea appears aimed at giving the regional group a boost after the league’s earlier mission was pulled out of the country because of security concerns.

Ban called the continuing violence “unacceptable” and added: “I fear that the appalling brutality we are witnessing in Homs, with heavy weapons firing into civilian neighborhoods, is a grim harbinger of worse to come.”