By Annie Barnard and Steven Lee Myers
BEIRUT, Lebanon — Fierce clashes broke out on Friday on multiple fronts in the northern Syrian city of Aleppo, according to the government and its opponents, as frustrated world leaders meeting in New York offered modest new pledges of support for those trying to overthrow the Syrian president, Bashar al-Assad.
Rebels mounted what they called a “decisive battle” for Aleppo, Syria’s largest city, after months of brutal but inconclusive fighting, aiming to seize control of predominantly Christian and Kurdish neighborhoods from which, the rebels say, security forces and pro-government militias have attacked.
Such a move could shift the military balance in the city, but also carries the risk of further inflaming ethnic and sectarian rifts. The government portrays itself as the guardian of minorities, and Christians and Kurds remain split on whether to support the uprising, dominated by the country’s Sunni Muslim majority.
Secretary of State Hillary Rodham Clinton, presiding over a meeting of foreign ministers and Syrian opposition leaders, announced that the United States was increasing its humanitarian assistance to those displaced by the fighting by $30 million, bringing the total to $130 million.
She also announced that an additional $15 million would go to support civilian opponents of Mr. Assad’s government working inside the country to establish an alternative government in areas now liberated from Syrian forces. That brings the amount the United States has given to the opposition to nearly $45 million, mostly in computers and communications equipment, even as it opposes providing arms to rebel fighters.
“In these places we are seeing the emergence of a free Syria,” Mrs. Clinton said at the meeting of a group that calls itself Friends of Syria, “and the United States is directing our efforts to support those brave Syrians who are laying the groundwork for a democratic transition from the ground up.”
While France and other nations also pledged more assistance, the leaders expressed frustration that international action remained blocked at the United Nations Security Council and that moral and financial support for Syria’s rebels had so far done little to stop the slaughter.
“We did not manage to change the situation,” said Nabil el-Araby, the secretary general of the Arab League, whose proposals for a resolution to the conflict have failed. “That’s why we believe that it is essential that we change our modus operandi.”
The officials, however, offered no new initiatives to do so.
In response to the unrelenting conflict and recent reports of new massacres in Syria, the United Nations’ human rights body voted on Friday in Geneva to strengthen and extend the term of a commission gathering evidence of abuses that could provide a basis for future prosecution by national or international courts.
The agency, the United Nations Human Rights Council, voted to continue the work of the Commission of Inquiry for six months and to expand its financing and its staff. Later Friday, the council’s president announced the appointment of two more commissioners — including Carla Del Ponte, formerly the chief prosecutor at the war crimes tribunal at The Hague — to join the two already on the panel, which is trying to map abuses committed since the uprising against Mr. Assad began in March 2011.
The resolution, presented by Morocco on behalf of a group of Arab countries, won the backing of 41 of the council’s 47 members. It was opposed by China, Cuba and Russia. Three other members, including India, abstained.
At the Pentagon on Friday, Defense Secretary Leon E. Panetta said some of Syria’s chemical weapons components might have been moved in recent weeks to more defensible locations, but all of the main chemical weapons depots were secure. Even so, it was the first official acknowledgment of even limited movements of Syria’s chemical weapons arsenal.
“The main sites, as we’ve determined and monitored, still remain secure,” Mr. Panetta said at a news conference. But he said the nation’s spy agencies could not answer all the questions about potential movements of Syria’s stockpiles of unconventional weapons. “There has been intelligence that there have been some moves that have taken place,” he said. “Where exactly that’s taken place, we don’t know.”
In Aleppo, both sides reported heavy fighting in the Sheik Maksoud neighborhood, a Kurdish district largely controlled by the Kurdistan Workers’ Party, which has long been allied with the Syrian government. The neighborhood is near two of the city’s main Christian areas, Midan and Sulaimaniya.
“It is our final war in Aleppo,” Abu Yasser, a senior fighter in the Free Syrian Army, the main rebel umbrella group, said in a telephone interview. “Anyone — Sunni, Shiite, Christian or Kurdish — who stands with the Assad forces will be a legitimate target.”
Rebels also said they had seized the mufti of Dara’a, the southern city where the uprising began, and forced him to leave Syria for Jordan because he had refused to join them. And in the Yarmouk refugee camp, home to Palestinian refugees and Syrians, opposition activists accused security forces of arresting scores of civilians, including children, who had fled earlier fighting and taken shelter in school buildings belonging to the United Nations refugee agency.
In Azaz, on the outskirts of Aleppo, rebels reported that government airstrikes had demolished buildings, killing several civilians, and posted online a video of a small boy’s body being pulled from rubble.
Anne Barnard reported from Beirut, and Steven Lee Myers from New York. Reporting was contributed by Hwaida Saad and Hania Mourtada from Beirut, Nick Cumming-Bruce from Geneva, and Thom Shanker from Washington.