U.N. Ambassador Jeane J. Kirkpatrick said yesterday the "assault" on thousands of Miskito Indians in Nicaragua by that country's Sandinista government is "more massive than any other human-rights violation that I'm aware of in Central America today."

Testifying before the Senate Foreign Relations subcommittee on western hemisphere affairs, Kirkpatrick said that while there were not many "model" regimes in Central America, Nicaragua "probably stands in first place as a human-rights violator" because of the "campaign of systematic violence" against Indians who have resisted incorporation into the leftist revolution.

Kirkpatrick's testimony echoed that of other administration officials who in recent weeks have focused on the plight of the Miskitos to strengthen their case that the Cuban-backed Nicaraguan government is a menace to other governments in the region and has turned out to be more repressive than the dictatorship of Anastasio Somoza, which it replaced.

This line of testimony, however, prompted Sen. Paul E. Tsongas (D-Mass.) to ask Kirkpatrick if the repeated invoking of the Indians to discredit Nicaragua is part of a much larger plan to blockade Cuba or Nicaragua with U.S. naval forces and shut off the flow of arms from these countries to rebels fighting the U.S.-backed government in El Salvador.

Tsongas said he, too, was concerned about how the Indians were being treated and that he also accepted that an unknown quantity of arms were flowing to the rebels, "but I happen to think something is going on."

As Tsongas speculated, things are deterioriating for the Salvadoran government, and President Reagan has committed himself personally to the effort to save the region from Havana-style takeovers. While there is no support for sending U.S. ground troops, Tsongas reasoned there is a reservior of anti-Castro feeling in this country that Reagan could call on, combined with the anti-Nicaragua campaign, to support a naval blockade to stop the arms flow. It could be carried out, he speculated, under the 1947 Rio Treaty, which provides for mutual security in the hemisphere.

Kirkpatrick said she "didn't know what to say" to the Tsongas theory, but that "obviously it has some merit" and that "anybody who thinks about the security of the hemisphere would be well-advised to think about" the Rio Treaty "as an instrument, a very important instrument, for maintaining security."

But she quickly added that there is no intention or desire on the part of the U.S. government or Salvadoran officials to have U.S. troops in that country and said she doesn't "think anybody is contemplating a blockade at this time."

"The United States," she said later, "is not going to war against anybody, that's quite clear, and we are not engaging in acts of war." Rather, she said, what strikes her as the best thing to do is to use U.S. "technological power," such as electronic and photographic monitoring techniques, to help choke off the flow of supplies.

Kirkpatrick maintained that in the 2 1/2 years since Somoza fell, "Nicaragua's hopes for greater freedom, democracy and security from government tyranny have very nearly died."

"Somoza was a perfectly clear-cut dictator," she said, "who took a very large share of what was worth having." But she said that under him there was more freedom to criticize the government than there is now, so that while the human-rights situation "was not good" under Somoza, "it almost surely was not as bad as the current one."