Pakistan said today it will go ahead with plans to purchase nuclear reprocessing plant from France despite opposition from the United States and Canada.

Aziz Ahmed, minister of defense and foreign affairs, told a news conference the transaction involves only Pakistan and France. "No third country has any right to demand that Pakistan should abandon the reprocessing plant," he said. He criticized Canada for suspending nuclear cooperation with Pakistan over the issue.

The plan, expected to cost about $150 million, is intended to reprocess nuclear fuel by extracting plutonium from spent reactor fuel rods.

The United States and Canada are opposed to acquisition of such reprocessing facilities because of concern that the plutonium extracted could be diverted to production of nuclear weapons.

U.S. officials reportedly have threatened to delay or bar sales to Pakistan of advanced American fighter planes unless Pakistan cancels its plans for the reprocessing plant.

Since the Pakistani-French deal was concluded, the French government has adopted a stricter policy on nuclear exports and a flat ban on future sales of reprocessing facilities. It said France would honor all contracts already signed, however, including the one with Pakistan. But French officials indicated at the time that they would not be unhappy if the Pakistanis bowed to U.S. pressure and canceled the deal.

Observers in Islamabad saw the announcement as a clear signal to the incoming U.S. administration that Pakistan would not submit to Washington on the issue. President elect Jimmy Carter has expressed concern about Pakistan's decision to buy the plant.

The nuclear supply situation on the Indian, subcontinent has been a major issue since India exploded a nuclear device in 1974. Canada canceled its cooperation with India and the U.S. Nuclear Regulatory Commission has submitted shipments of fuel to India to long consideration, granting one limited license but holding up a second and larger shipment.

The Soviet Union, however, agreed last month to supply India with 200 tons, or about a six-year supply, of heavy water for it nuclear program. Ahmed disclosed his government's stance on the reprocessing plant deal in discussing the decision announced by Canada Dec. 23 to end it nuclear cooperation program with Pakistan.

Under the program, Canada built 137-megawatt nuclear power plant at Karachi, 900 miles south of Islamabad, and also agreed to supply uranium fuel and spare parts for the facility.

Ahmed said Pakistan would try to purchase fuel and spare parts from other nations, adding that new conventional generating facilities using steam and natural gas-fired turbines would be built by July to provide standby electrical power for Karachi.Reliable sources estimated that Pakistan has a year's supply of nuclear fuel for the Karachi plant.

Ahmed said the arbitrary Canadian action violated bilateral ageements because Pakistan scrupulosuly has honored its undertakings not to use Canadian nuclear supplies for any military purpose.

"It (Canada) demanded that we accept some totally unreasonable conditions as the price for Canada's continuing cooperation with us while pleading helplessness to get India to accept similar conditions," he added.

Ahmed said Canada had wanted Pakistan to accept Canadian safeguards for Pakistan's entire nuclear program and not only for facilities provided by Canada.

Also, he said the Canadians had sought to require Pakistan to be bound by its commitments under the Canadian agreement during the operating life of the Karachi nuclear plant even if Canada terminated its nuclear prgram with Pakistan.

"These two demands were so unreasonable and one-sided that Pakistan could not but reject them," Ahmed said. "Canada has no right to make any of these demands in the first place."