Iraq is using its position as an oil exporter to "blackmail" Western European and other oil-dependent nations into providing it with the supplies, technology and expertise necessary to produce nuclear weapons, Sen. Alan Cranston (D-Calif.) charged yesterday.

In a speech on the Senate floor, Cranston, the assistant minority leader, said Iraq has embarked on a crash program to develop nuclear weapons with a "Manhattan Project-type approach" and has gained assistance by placing pressure on Portugal, Brazil, France and Italy.

Cranston also charged that Pakistan has continued to purchase "dual use" nuclear technology in Europe that can be used to fashion weapons and that "it will most likely have the capability and the materials for fabricating a number of nuclear weapons by the end of 1982."

He said that in response to these developments the United States should threaten to cut off nuclear supplies it provides to France, Italy and other countries unless they halt nuclear cooperation with Iraq and Pakistan.

Cranston did not disclose the basis for his charges but said he had verified his information regarding the Iraqi and Pakistani nuclear programs with officials of the Reagan administration and was satisfied that it was accurate.

Asked about Cranston's speech, Secretary of State Alexander M. Haig Jr., who was on Capitol Hill yesterday, said, "I make it a habit of not commenting on something I haven't read or studied. I'll have to do that."

Cranston said there is no evidence that Iraq is actually designing a nuclear bomb, adding that Iraq appears to be about five years behind Pakistan in developing nuclear weapons. But he said Iraq was "demonstrating graphically the danger that radical oil powers will use the 'oil weapon'" to gain access to nuclear weapons technology.