The United States yesterday rejected recent Soviet attacks as being "without foundation" and challenged Moscow to negotiate seriously on arms reduction.

If the Soviet Union is sincere about reducing the risk of war, State Department spokesman John Hughes said, "it should concentrate its efforts on negotiating seriously in Geneva toward real arms reduction agreements."

The U.S. statement came after a series of attacks by Soviet leaders, including President Leonid I. Brezhnev, accusing the United States of "aggressive action" that threatens to "push the world into the flames of nuclear war."

Against the backdrop of increasingly harsh statements from Moscow, yesterday's U.S. response was relatively mild. It appeared to reflect a decision by Washington to adopt a posture of reasonableness as Western Europe approaches critical decisions on nuclear issues and attempts are made to patch up alliance relations.

Those relations have been strained in recent months in part because of the continuing split over the Reagan administration's gas pipeline sanctions. And some alliance strategists have expressed fears that the strains could affect decisions by Italy and West Germany to begin accepting new medium-range nuclear missiles in late 1983.

Italian Prime Minister Giovanni Spadolini said last week during a visit to Washington that Italy would stand by its commitment to take the new missiles. But he stressed the importance of pursuing the current round of arms talks in Geneva.

The new missiles are designed to counterbalance Soviet medium-range missiles and to assure Europeans that the United States would be involved immediately in event of a Soviet attack.

Negotiations on the pipeline issue continued yesterday at the State Department and a U.S. official indicated progress continued slowly but added that "we are not at the end of the road yet." Another meeting was set tentatively for today.

A State Department official said yesterday that U.S. analysts believe Moscow is "trying to play on public opinion here and in Western Europe to undo defense programs and new missiles. They really seem to have hope of cutting off the new missiles."

In a separate statement, Hughes also said yesterday that the United States and its NATO allies had agreed to focus attention on Poland and the "deteriorating Soviet human rights record" when the Madrid review of the European Security Conference reopens today. "Clearly there can be no return to 'business as usual' at Madrid."

There had been fears as recently as a month ago that the United States and major Western European countries would be split sharply over the Madrid talks, adding yet another serious irritant to the alliance.

"At least there is agreement on a starting strategy," one State Department official said. But he warned that "unless we get satisfaction on existing problems, such as Poland, we can't go on to new business."

A number of Western European countries are believed to be anxious to push ahead on negotiations on proposals for a disarmament conference designed to lessen chances of surprise attack in Europe.