Israeli Prime Minister Menachem Begin today stepped up his counter-attack against U.S. condemnation of the Israeli bombing of Iraq's nuclear reactor, sending a sharp rebuke to U.S. Defense Secretary Caspar Weinberger and warning his political opponents at home that criticism of the raid encourages "all those who are plotting against us."

The prime minister did not release the letter to Weinberger, but it was understood to say that Begin is hoping for greater understanding by the Reagan administration for Israel's security needs.

Begin accused the defense secretary Thursday of demanding that the United States withhold from Israel "all economic and military aid." Weinberger denied making any such recommendation.

Also, in a message prepared for a pro-Israel parade in New York today, Begin reiterated his contention that the bombing of the reactor was a legitimate act of self-defense and had to be done last week because the reactor soon was to become "hot" and could not be destroyed later without inflicting heavy civilian casualties in Baghdad.

Aides to the prime minister stressed that Begin is not attempting to appeal to U.S. public opinion over the head of President Reagan, but that he had been asked to send a message to the New York gathering and that it was only natural to deal with a topical issue like the bombing raid controversy.

Begin also denounced the opposition Labor Party for creating "deliberately false propaganda, which is being exploited by all those who are plotting against us regarding the glorious operation of the Israel defense forces in demolishing the automic reactor near Baghdad."

Meanwhile, the Foreign Ministry acknowledged today that it could not confirm reports of a threat against Israel that Begin has repeatedly attributed to Iraqi President Saddam Hussein to justify the bombing.

Begin on several occasions reported that Hussein, in the Oct. 4 issue of the Baghdad daily, Al Thawra, said, "The Iranian people should not fear the Iraqi nuclear reactor, which is not intended to be used against Iran, but against the Zionist enemy."

Foreign Ministry researchers reported that the quote cannot be found in Al Thawra, and they are attempting to determine where it originated.