The Senate voted unanimously and without debate yesterday to authorize $15 million in direct humanitarian aid to rebels fighting the Soviets in Afghanistan.

In two more rare shows of bipartisanship, the Senate also agreed unanimously to authorize an additional $1.5 billion in economic aid for Israel for fiscal 1985, and to withhold some aid to nations that do not curb their illegal drug trade.

The three amendments to the $12.8 billion foreign aid bill for fiscal 1986 had been expected to generate more than the few minutes of discussion they aroused in a near-empty chamber. Afterward, the Senate moved quickly on several other amendments.

The bill itself is expected to come to a vote today; the House has not yet acted on its version of the legislation.

Senators refused on a 52-to-41 vote yesterday to eliminate a multiyear authorization for the Reagan administration's package of aid to Central America.

Then they refused, 60 to 33, to delete a provision of the bill that would allow countries for the first time to spend their U.S. aid to reimburse landowners for property the government seizes. Such spending previously had been prohibited under a law authored by Sen. Jesse Helms (R-N.C.), who sponsored yesterday's unsuccessful effort to retain it.

Proposing the aid to the Afghan resistance, Sen. Gordon J. Humphrey (R-N.H.) said there were "few issues on which the members of this house are more united." The measure would channel food, clothing, medicine and other humanitarian aid to the rebels in Afghanistan through private volunteer organizations, Humphrey said.

Cosponsor Claiborne Pell (D-R.I.) said he hoped the aid would "serve to stem the flow of refugees out of Afghanistan by . . . encouraging them to remain within their country."

The State Department already has quietly provided $4 million in nonlethal aid to the Afghan resistance and is in the process of sending $4 million more. Officials testified last week that more aid would be welcome, but congressional sources said State Department lobbyists had objected to Humphrey's requirement that "not less than" $15 million be spent, saying that would tie the president's hands.

The administration has not yet formally requested the increased aid to Israel, insisting that country must first take concrete steps toward structural reform of its bloated and inflationary economy.

But, in offering the aid package, Foreign Relations Committee Chairman Richard G. Lugar (R-Ind.) told the Senate, "We now have indications from the Department of State that these are the levels the administration would like to have."

He said the funds would recognize "positive steps" Israel has taken, such as cutting its bread subsidy and increasing utility and gas tariffs.

The amendment is in addition to $1.2 million in economic aid and $1.8 million in military assistance already provided this year. It also would authorize another $500 million for Egypt in fiscal 1985, $82 million for regional programs and $1.2 million to help refugees along the Gaza strip.

The members also approved a proposal by Sen. Lawton Chiles (D-Fla.) to cut part of the aid package to Bolivia and Peru and to consider reducing aid to other countries if the Drug Enforcement Administration reports that their drug trade has not slowed.