ISTANBUL — Syrian government forces shelled rebel strongholds across the country on Sunday, opponents of the government said, while the main opposition group in exile, the Syrian National Council, chose a new leader.

The shelling struck at targets in and around Homs in central Syria and near al-Heffa, just east of the port of Latakia, as well as other locations, opposition groups said. The violence continues despite a United Nations-brokered cease-fire, which has so far had little effect on the conflict, and the presence of international monitors.

The new leader of the council, announced on Sunday in Istanbul, is Abdelbaset Sieda, a Kurdish professor of Arabic and philosophy who promised the organization would be overhauled.

“We will expand and extend the base of the council,” he told reporters at a news conference, “so it will take on its role as an umbrella under which all the opposition will seek shade.”

The Syrian National Council, formed last fall, has been plagued by infighting and has been criticized as ineffective, amounting to little more than a front for the long-exiled Syrian branch of the Muslim Brotherhood with little influence inside the country. Its top post was supposed to rotate every three months, but Bourhan Ghalioun, another exiled academic, held on to it until an outcry over his most recent re-election in May, especially from inside Syria, prompted him to step down.

Council members involved in the process hailed Mr. Sieda’s election as proof that the Syrian opposition was committed to upholding democratic principles and the idea of a “leaderless revolution.” He ran unopposed.

“The ideal leadership of the council is not through one person — because no one is elected and has actual legitimacy,” said Bassma Kodmani, a member of the executive committee. Until such time as there are free elections in Syria, she said, the choice of the president of the council should be made by consensus.

“The revolution does not want to see a big leader, or one individual who leads everything,” Ms. Kodmani said. “Personalization leads to polarization.”

Still, critics both in the wider membership of the council and outside the group said Mr. Sieda had emerged as the consensus choice precisely because he represents no one, either inside Syria or out. Both the Muslim Brotherhood and liberals in the council concluded that he did not pose a threat or provide an advantage to any bloc within the council, they said, but for the same reasons he will have little real authority, and the bickering will continue.

“The Muslim Brotherhood, especially, does not want a strong person, neither someone with political strength nor a strong personality,” said Hasan Kasem, a young liberal activist.

Mr. Sieda, who turns 56 on Tuesday, has lived in Sweden for the past 17 years, and calls himself an independent. As a Kurd, he belongs to a minority that was oppressed for years by the Syrian government. Most Kurdish opposition groups do not take part in the council because it has not promised to back a federal government structure for Syria that would give Kurdish areas some measure of autonomy.

At the news conference, Mr. Sieda defended his long record of opposition to the governments of President Bashar al-Assad and his father, Hafez. To counter criticism of the council, he noted that the executive committee had just added three members with experience running local leadership councils who had fled Syria recently. Answering complaints that not enough humanitarian aid was being sent to areas affected by the conflict, Mr. Sieda said the council would soon distribute $3 million worth of aid inside Syria.

Sunday was the sixth straight day that al-Heffa and neighboring villages were hit with rocket, mortar and tank shelling, opposition groups said. But the official news agency, SANA, gave a different account, saying that “armed terrorist groups” — its label for all opponents of the government — looted and burned public property in al- Heffa and killed residents.

Farther south in Qusair, near the border with Lebanon, activists said an attack by government forces killed at least six people on Sunday. They said that Homs, about 20 miles away, also sustained heavy bombardment for the third straight day, with several dozen people killed in the area over the weekend.

“Each day is more than the next,” said Salim Kabani, an activist reached via Skype. “It is the heaviest since the beginning of the revolution, and includes all kinds of weapons — artillery, mortars, helicopters.”

Mr. Kabani said that all the soldiers at a small military base in a village called Ghanto near Homs defected to the rebel side on Sunday, and that government helicopters bombed the base soon after the soldiers fled.

Claims from either the opposition or the government about events in Syria are difficult to confirm independently, because the Syrian government sharply limits the number of foreign journalists allowed in to the country and where they can go. Hwaida Saad and Sebnem Arsu contributed reporting. (Copyright 2012 The New York Times Company 06/11/12)