WASHINGTON (AP) _ A senior official said Tuesday the Reagan administration is pushing ahead with plans for speedy delivery of humanitarian aid to Nicaragua's Contras, but the Sandinistas claim U.S. assistance to the rebels would be illegal.

Alan Woods, administrator of the Agency for International Development, said plans are being prepared for air drops of the food and other aid approved for the rebels last week by Congress.

The temporary cease-fire agreement signed by the leftist Sandinista government and the Contras allows for deliveries of humanitarian aid to the rebels.

Woods acknowledged at a news conference that Sandinista cooperation is required and said the issue is being discussed between government and rebel negoitators.

''We would strongly hope that the Nicaraguan government would be sensitive ... and prepare to allow humanitarian assistance - food, medical supplies, clothing and shelter - to move without interruption,'' Woods said.

But Nicaraguan President Daniel Ortega on Monday called the U.S. aid package that was approved by Congress illegal, saying ''we cannot accept any type of aid to the Contras because it would be legitimizing the interventionist policy of the United States.''

Woods said preparations are being made for deliveries to rebels deployed outside Nicaragua - presumably in Honduras - within the next few days. Contras based inside Nicaragua will receive aid once they move to cease-fire enclaves agreed to in ongoing negotiations.

The $49.7 million aid package calls for $17.7 million to be sent to the Contras and a similar amount to be allocated to help children who are war victims.

Woods said private contractors will be hired to carry out shipments to the rebels by air, while private voluntary organizations are being contacted to send experts trained to help children in need.

He said AID will abide by a provision in the Sandinista-Contra agreement that only ''neutral organizations'' be allowed to take part in delivery of the assistance.