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Harvard is being sued over claims that a crackdown on single-sex social clubs at the Boston-area university amounts to sexual discrimination.

A number of sororities and fraternities filed lawsuits on Monday challenging a policy Harvard adopted in 2016.

Under the policy, students who join single-sex clubs cannot receive endorsement letters from college deans for postgraduate fellowships.

Harvard said the aim was to help end practices of exclusion at the school.

But the lawsuits allege that Harvard, one of the top-rated and most selective universities in the world, is discriminating against students on the basis of their sex by punishing men and women who join all-male or all female-organisations.

"Harvard's sanctions policy seeks to dictate the sex of people with whom men and women may associate and the gender norms to which men and women must conform," the federal complaint reads.

Among other sanctions, the policy states that members of single-sex clubs are barred from serving as captains of sports teams or leaders of other official student clubs.

A separate complaint in a state court alleges that the Harvard policy violates Massachusetts law.

A lawyer for the sororities and fraternities, Stanton Jones, said in a statement that Harvard "should get out of the business of trying to dictate who students spend their time with off campus", Reuters news agency reported.

In October, a case was brought against Harvard by a group claiming that the admissions office discriminated against Asian applicants in order to boost other racial groups.

That lawsuit alleges that the university illegally weighs race when considering an applicant, forcing Asian-Americans to meet a higher bar.