Swollen rivers began to recede in parts of Pakistan on Thursday as the international community announced more aid, offering some tenuous hopes to millions of Pakistanis affected by weeks of record flooding. But waters were rising in Sindh Province, in the south, and the United Nations warned the humanitarian disaster remained dire, and said the number of people in need of medical care, emergency shelter and food and drinking water continued to grow. Senator John Kerry of Massachusetts, visiting Pakistan to assess the damage and relief efforts, said the United States would increase its flood aid to $150 million. He said American officials did not want the floods to strengthen the position of Islamic extremists. Some hard-line Islamic charities have been providing aid to flood victims as the government’s efforts have faltered. “All of us are cooperating to deal with insurgency and violence,” Mr. Kerry told a gathering of United States Marines and Pakistani troops at Ghazi Air Base. “This is an additional test.” The Asian Development Bank, meanwhile, said it would loan Pakistan $2 billion to help the country rebuild from the floods, which have covered at least one-fifth of the country and claimed as many as 1,600 lives. “We’re doing this because we want to show we’re taking this seriously,” said Juan Miranda, the bank’s director general for Central and West Asia.