ALEPPO, Syria - Syrian army tanks shelled Aleppo on Sunday and a helicopter gunship strafed rebel positions with heavy machinegun fire as they fought for control of the country's biggest city and key battleground of the 17-month uprising.

After UN Security Council paralysis on Syria forced peace envoy Kofi Annan to resign last week, and with his ceasefire plan a distant memory, rebels have been battered by the government onslaught in Aleppo and the capital Damascus.

A Reuters correspondent in Aleppo witnessed fierce street fighting in the Salaheddine district, a gateway into the city of 2.5 million people.

Tanks pounded alleyways where rebels sought cover and one shell hit a building next to the reporter, pouring rubble onto the street and sending huge billows of smoke into the sky.

State television said Assad's forces were "cleansing the terrorist filth" from the country, which has been sucked into an increasingly sectarian conflict that has killed some 18,000 people and could spill into neighboring states.

In Damascus, jets bombarded the capital on Saturday as troops kept up an offensive they began a day earlier against the last rebel bastion there, a resident said.

Both cities - vital prizes in the battle for Syria - were relatively free of violence in the early months of the uprising but fighting flared in Damascus shortly before a July 18 bomb killed four of Assad's inner circle. It later erupted in Aleppo.

In face of the ongoing crisis, US Secretary State of State Hillary Clinton will travel to Istanbul next week to hold talks with the Turkish government on the situation in Syria, a State Department spokeswoman said on Sunday.

"Secretary Clinton goes to Istanbul for bilateral consultations with the Turkish government on Syria as well as to cover other timely issues," spokeswoman Victoria Nuland said in a statement sent to reporters during a visit by Clinton to the southern African nation of Malawi.

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Clinton's planned talks in Istanbul on Aug. 11 will form part of renewed international efforts to tackle the escalating crisis.

Prospects over the possibility of a negotiated solution have dimmed since United Nations peace envoy Kofi Annan resigned this week complaining of paralysis in the UN Security Council over the Syria peace efforts.

On Saturday, a rebel commander in Aleppo said he expected a Syrian army attack on rebels "within days", echoing the head of the UN peacekeeping department who said there had been a "considerable build-up of military means."

"We know they are planning to attack the city using tanks and aircraft, shooting at us for three to four days and they plan to take the city," Colonel Abdel-Jabbar al-Oqaidi said.

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