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The United Nations made the largest humanitarian appeal in its history Friday, sharply increasing its estimate for the funds needed for Syria, as rights groups said it must do more to ensure its aid cannot be used by President Bashar al-Assad as a weapon of war.

U.N. humanitarian chief Valerie Amos said that with half the Syrian population — more than 10 million people — expected to need aid by the end of the year, the United Nations and its partners will require at least $5.2 billion for projects in Syria and neighboring countries, up from a previous estimate of $3 billion. But activists say the world body’s reach in rebel-held areas is limited, given the government’s control of humanitarian access.

Amos said the situation in Syria has “deteriorated drastically,” with violence intensifying in most areas of the country. According to U.N. estimates, more than 4 million people have been internally displaced during the two-year-long conflict, and schools and hospitals have been among the military targets.

The battle for the central border town of Qusair, which ended Wednesday when Syrian army troops backed by guerrilla fighters from the Lebanese militant movement Hezbollah seized it from the rebels, has triggered a new wave of displacement in the area. Bolstered by their victory, pro-government forces pressed on with an offensive to clear pockets of resistance around the town Friday, seizing control of two nearby villages to which many civilians and fighters had fled.

The Syrian state news agency said that the villages of Salhiyeh and Masoudiyeh had been cleared of “terrorists” — the governments term for the rebels — and activists said army troops were nearing the village of Buwaida, where they said hundreds of civilians had taken refuge, along with rebels.

The first food aid to reach Qusair, which pro-government forces had besieged for weeks, was distributed Friday by the World Food Program through the Syrian Arab Red Crescent. The Syrian government had blocked earlier efforts to assist in the town.

“Assad uses aid as a weapon of war: Everything goes through Damascus, so they can control where aid is delivered and punish rebel-held areas by blocking aid,” said Peter Bouckaert, emergency director at Human Rights Watch. It is time, he said, for the United Nations and the international community to “demand to be allowed to carry out cross-border operations.”

In addition to the internally displaced, more than 1.6 million documented Syrian refugees have flooded into neighboring countries, putting enormous pressure on their governments. But aid organizations say they are constrained in their efforts to help by huge gaps in funding. The previous U.N. appeal for $3 billion has drawn just $1.4 billion.

In Lebanon, which has a population of 4 million, some 500,000 arrivals registered or awaiting registration by the U.N. refugee agency are upsetting the country’s fragile balance. Tented shanty towns have popped up along roadsides as new arrivals struggle to find accommodation.

Also Friday, rebels seized control of the Minnegh air base north of Aleppo after a weeks-long siege, the Britain-based Syrian Observatory for Human Rights reported. But intense clashes continued in the area — the scene of a recently escalated government offensive on Sunni towns — as the Syrian air force carried out strikes and two Scud missiles landed near the base.