MASHHAD, Iran - Trucks carried wheat flour into Afghanistan on Tuesday in the first major shipment of food aid across the Iranian border, a U.N. aid official said.

The 100-ton donation from Britain was taken by Afghan truckers toward the northwestern city of Herat, about 65 miles from the Iranian border town of Dogharoun, said Marius de Gaay Fortman, a World Food Program coordinator.

The shipment came after more than a week of negotiations with Afghan drivers and could open the way for an aid corridor into western Afghanistan. Relief supplies have already moved into other parts of Afghanistan from Pakistan.

"We are able to bring the food to the people rather than have the people come to the food," Fortman said in the northeastern city of Mashhad.

The WFP plans to send more than 1,000 tons of wheat flour across the border in coming days, he said.

UNICEF, the U.N. agency for children, also planned to send a convoy to Herat from Iran later Tuesday but said it would not send any new supplies into Afghanistan from Pakistan because of security concerns.

The seven-truck UNICEF convoy contains health kits for up to 350,000 people, blankets, jerry cans, water purification tablets and drugs to treat respiratory infections, spokeswoman Wivina Belmonte said in Geneva.

"Every day closer to winter the needs become more urgent. We know the catastrophic effects of winter on children and we have to be prepared," said Belmonte.

The World Food Program also has more than 50 tons of high-protein biscuits and other supplies have been stockpiled at the border in anticipation of waves of refugees.

So far, however, there is no sign of a mass movement of people toward Dogharoun, officials said.

Iranian authorities have closed its 600-mile frontier with Afghanistan and say it will attempt to aid any refugees in camps on the Afghan side.

The U.N. refugee agency has appealed to Iran to open the border. But Iranian officials say the country already has more than 2 million Afghans and cannot handle any more refugees.

Separately, two C-17 transport jets returned to the Ramstein Air Base in Germany about noon Tuesday after dropping a total of around 35,000 humanitarian daily rations, said Maj. Scott Vadnais, a spokesman for U.S. Air Force Europe.

He said it was "another successful mission in that we did the drop at the right time and in the right place." The aid was dropped in eastern Afghanistan.

Vadnais declined to say when further missions would be flown.