Secretary of State Alexander M. Haig Jr. said yesterday that new routes for arms to the Salvadoran insurgents have been opened and that there is "some evidence" their arms supply is increasing again.

Haig made the statement in testimony to a Senate Appropriations subcommittee one day after the State Department, acknowledging intelligence reports of such new arms infiltration routes, said they have "not resulted to date in a significant increase in the supply available to the guerrillas."

The alternate supply paths, Haig said, are intended to compensate "for direct air routes" of supply. The Reagan administration previously charged that large amounts of military equipment were supplied to Salavadoran rebels via air shipments through Nicaragua late last year, and in recent weeks the administration reported that this supply has largely ceased.

"There has been some improvement" in the military situation in Salvador, Haig reported, adding that "the insurgents are bascially on the defensive today."

Officials familiar with intelligence from the area said some and other supplies reportedly have been received by Salvadoran rebels overland through Honduras and Guatemala for many months. While the intelligence reports are varied and imprecise, the officials said, there was little indication that this continuing flow rivals the amounts of arms received through Nicaragua.

In the Senate hearing, Haig defended recent U.S. policies toward Cuba in the face of a claim by subcommittee chairman Sen. Lowell P. Weicker (R-Conn.) that the attempt to isolate Cuba had only increased Soviet influence there.

"I think Mr. Castro has a choice to make," Haig said. "If he is going to persist in the provision of arms, terrorist training, strategizing for a forcefull overthrow of existing regimes in the hemisphere, it is going to be extremely difficult if not impossible to initiate a dialogue with him."

On the other hand, Haig held out the prospect of "remarkable and astonishing" benefits for the people of Cuba if relations with the United States are repaired. He said Cuba today is "an economic shambles" despite receiving at least $8 billion to $10 billion annually from the Soviet Union.

Haig said the administration would welcome a dialogue and improved relations with Cuba but that "it must be preceded by some indications that the leadership in Cuba is willing to abide by the rules and the standards of international law."

Weicker, on other hand, argued that in fact "Cuba is not isolated" but has good relations with many close allies of the United States, as well as the hemispheric neighbors of Canada and Mexico. "A name-calling policy and creation of a vacuum has only succeeded in putting the Soviet Union smack into our hemisphere," Weicker said.

Under questioning from other senators, Haig defended President Reagan's decision late last week to lift the partial grain embargo against the Soviet Union. The secretary of state Haig said the overriding factors was Reagan's campaign promise to lift the embargo and that spotty adherence to the embargo by U.S. allies was another problem, he said.