Syrian rebels virtually cut off roads to Aleppo from neighbouring Raqa province on Monday, severing regime supply lines, as France announced it had earmarked financial aid for the opposition coalition.
Reflecting its growing recognition since being formed earlier this month, the National Coalition said it had named an "ambassador" to London, its second such appointment, following an invitation to do so by the British government.
The Red Cross, meanwhile, called on both sides in the conflict to respect international humanitarian law, a day after an aerial bombing killed 10 children.
On the ground, the insurgents took full control of Tishrin dam on the Euphrates river after days of fighting, an area that connects the northern provinces of Aleppo and Raqa, the Syrian Observatory for Human Rights said.
A resident of nearby Manbij confirmed the report, adding that employees of the hydropower dam were continuing operations.
"The capture of the Tishrin dam is very important. It means that the army basically has only one road left to Aleppo," Observatory director Rami Abdel Rahman told AFP.
"The highway crossing over the Tishrin dam was the last main route from Raqa province under regime control."
The rebels now hold sway over a wide expanse of territory between the two provinces bordering Turkey, which backs the revolt against President Bashar al-Assad.
The army must now rely on the Damascus-Aleppo highway to bring reinforcements to embattled commercial hub Aleppo, where fighting is deadlocked.
Rebels also gained full control of Marj al-Sultan air base east of Damascus, said the Observatory.
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Further north, a warplane launched bombing raids on a rebel command centre in Atme, two kilometres (1.2 miles) from the Turkish border, without causing casualties or hitting its target, an AFP journalist reported.
In a sign of growing confidence, rebel officers have formed a commission to lay the groundwork for a future army and liaise with the political opposition on issues such as arming fighters on the ground, a spokesman said.
He said the Free Officers Assembly would seek "to lay the correct foundations for the construction of the new Syrian army, which will be non-partisan," working with the opposition National Coalition.
The coalition on Monday named human rights activist and former teacher Walid Safur as its "ambassador" in London, according to a statement published on its Facebook page, after a similar appointment in France last week.
Safur set up the Syrian Commission for Human Rights in 1986 and was imprisoned several times, before moving to London, where he represented the Muslim Brotherhood.
France said it had allocated 1.2 million euros ($1.5 million) in emergency aid for the coalition, as Russian Prime Minister Dmitry Medvedev slammed Paris's support of the rebels as "unacceptable".
"France, which was first to recognise the coalition as the sole legitimate representative of the Syrian people, now wants to help it come to the aid of its countrymen in distress," Foreign Minister Laurent Fabius said.
Faced with an increasingly offensive revolt, the regime has been reducing its territorial ambitions to focus on Damascus, central Syria and Alawite bastions, as it digs in for a long war, analysts say.
Troops have been bombing rebel positions on the outskirts of the capital, including in Daraya, the site of the worst massacre in the 20-month conflict, with state media saying on Monday that troops had inflicted heavy losses on "Al-Qaeda terrorists" in their advance.
The International Committee of the Red Cross called on both sides to respect international and humanitarian law, a day after an aerial bombing killed 10 children in Deir Assafir, south of the capital.
An initial toll from the Observatory, which relies on a network of activists and medics for its information, said 34 people were killed on Monday. The watchdog has recorded a total of more than 40,000 deaths in the Syrian conflict.