Secretary of State Hillary Rodham Clinton said Thursday that Syrian opposition forces will become “increasingly capable” of carrying out offensive operations against the government of President Bashar al-Assad.

“They will, from somewhere, somehow, find the means to defend themselves, as well as begin offensive measures,” Clinton said. She stopped short of endorsing arming the opposition or signaling that the United States might take that step.

But Clinton’s comments moved the Obama administration closer than ever before to expressing support for supplying weapons and equipment to the Syrian opposition. Her remarks notably also did not include any of the previous warnings about the negative consequences of further militarizing the conflict.

Clinton spoke at a news conference after meetings here with European and Arab allies and partners, many of whom have pushed for stronger action in the face of a Syrian government crackdown in which at least 6,000 people are estimated to have died. The world leaders were in London en route to a “Friends of Syria” conference to be held Friday in Tunis.

Countries in the region, including Qatar, the United Arab Emirates, Saudi Arabia and Turkey, have advocated more support for the self-styled Free Syrian Army, formed largely of government military defectors. Although U.S. officials said they are unaware of any official arms shipments so far, regional governments are said to be facilitating the transfer of supplies bought on commercial markets.

One official, among several who spoke on the condition of anonymity about policies that are still under intense internal and international discussion, compared the situation to the one in revolutionary Libya and recalled that weapons supplies to opposition forces there began in a similar manner, followed by open shipments by some Arab countries. The U.S. government, he recalled, sent nonlethal military aid, including communications equipment, and supplied training for the rebel forces.

NATO, which spearheaded the military intervention in Libya that led to the ouster of Moammar Gaddafi, has said that it has no intention of intervening in Syria, where the popular uprising began nearly a year ago. And officials from countries leading the rhetorical charge against Assad have been uniformly leery of direct military action.

In the Republican presidential debate Wednesday night, candidates Mitt Romney and Newt Gingrich criticized the Obama administration for not endorsing efforts to provide weapons aid.

“We need to work with Saudi Arabia and with Turkey to say, ‘You guys provide the kind of weaponry that’s needed to help the rebels inside Syria,’ ” Romney said.

The primary public focus of the Tunis meeting will be on developing a unified effort to deliver humanitarian aid to Syria. But although supplies for the poorly armed opposition forces are unlikely to be mentioned in the communique being prepared, they will certainly be part of the closed-door conversations there.

The Tunis gathering is modeled on the “Friends of Libya” meetings that helped organize the international response to the situation there. Additional conferences are tentatively scheduled for March, in Istanbul, and April, in Paris.

“These meetings tend to focus the mind and jell” uncoordinated efforts, a senior State Department official said.

Representatives of more than 70 governments are expected in Tunis, along with the opposition Syrian National Council — modeled on the transitional governing council that eventually took power in Libya. But while Clinton called the SNC a “credible” representative of the Syrian people, international backers are acutely conscious of the exile-dominated group’s shortcomings, including its failure to win widespread support among Syria’s minority ethnic and religious groups and the lack of a viable transition strategy.

The United States and its partners, said a second State Department official, hope “to work with the opposition on implementing a pragmatic, practical transition plan that disproves Bashar al-Assad’s theory that the only alternative to him is chaos and civil war.”

A draft of the Tunis communique calls for stepped-up support to turn the SNC and other groups into a unified opposition and repeats international calls for Assad to allow humanitarian aid to Homs and other cities that have been under relentless government attack, according to officials from participating governments.

U.N. seeks to build pressure

The United Nations announced Thursday the appointment of former U.N. secretary general Kofi Annan as joint special envoy of the United Nations and the League of Arab States on the Syria situation.

A U.N. statement said Annan would be “guided” by a U.N. General Assembly resolution that last week endorsed an Arab League plan for Assad to step down. An earlier U.N. Security Council resolution endorsing the plan was vetoed this month by Russia and China.

“The pressure will build on countries like Russia and China,” Clinton said Thursday, “because world opinion is not going to stand idly by.”

In Geneva, a U.N.-appointed commission said Syria’s top military commanders and government officials have committed “widespread, systematic” rights violations that constituted crimes against humanity. The findings were based on interviews with hundreds of victims, witnesses, defectors and others.

“The government has manifestly failed in its responsibility to protect the populations,” the three-member U.N. Commission of Inquiry said in its report, which was presented to the U.N. Human Rights Council. The commission also gave the council a sealed list of individuals and military units suspected of bearing the greatest responsibility for the atrocities.

One of the commissioners, reached in Ankara, Turkey, called for an end to the violence but insisted that military intervention should not be an option.

Although Turkey has been in the forefront of calls for international action against Assad, its position is a difficult one. Turkey has welcomed refugees from the Syrian violence, and its refugee camps along the border are considered ideal for funneling humanitarian aid, and arms, to the opposition.

But Ankara is concerned that overt assistance to opposition military forces will spark trouble from Kurdish militants, whose population also spans the Turkish-Syrian border.

Assad’s onslaught continues

Syrian military forces Thursday kept up their weeks-long bombardment of Homs, the opposition stronghold where hundreds have reportedly been killed, including two Western journalists who were slain Wednesday and whose bodies remain in the city.

An artillery onslaught continued to target the embattled Baba Amr neighborhood in Homs, with activists reporting that government forces had surrounded the strongly anti-Assad area, prompting fears of a ground offensive.

At least three people were killed in Baba Amr on Thursday, according to a Syrian activist based in Beirut who spoke on the condition of anonymity because he feared for his safety. The activist said that others were buried under buildings and that the shelling was too intense to search for them.

Rami Abdulrahman of the London-based Syrian Observatory for Human Rights said that 23 civilians were killed in military operations in the city of Hama and that 11 were killed elsewhere.

Staff writer Colum F. Lynch at the United Nations and correspondent Alice Fordham in Beirut contributed to this report.