Nearly 20 Democratic senators and 80 members of the House of Representatives signed sharply-worded letters that were sent to secretary of state Mike Pompeo on Thursday, calling on his department to reverse a policy that has been used to deny citizenship to children of Americans born abroad through assisted reproductive technology.

The two letters, which criticised the State Department’s position as “cruel,” “offensive” and “deeply disturbing,” are the latest escalation in tensions over a long-standing citizenship policy that has recently come under scrutiny for its effect on same-sex couples.

Under State Department policy, children born abroad must have a biological connection to an American parent to receive citizenship at birth. That is not a problem when couples have babies the traditional way but can prove tricky when couples — particularly gay and lesbian couples — have children through techniques like surrogacy and in vitro fertilisation (IVF).

The focus on a biological connection also means that the department classifies certain assisted reproductive technology cases as “out of wedlock,” even if the parents are legally married, a designation that triggers specific requirements for passing on citizenship.

In several examples, the children of same-sex couples have had their citizenship denied or called into question, even though one or both parents are American.

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The State Department is fighting lawsuits from two of the couples, who argue that the policy discriminates against same-sex couples and their children by failing to recognise their legal marriages.

“Even in the face of the mounting hardship the policy has created for loving families, your Department has gone to great lengths to continue to defend a policy in federal court that separates American families before they reach the US border edge,” the senators said in their letter to Mr Pompeo, which was reported by The Daily Beast.

“This appears to be a thinly-veiled attack on LGBTQ Americans,” the letter added.

The effort, which coincided with the celebration of Pride Month in June, was led by senators Jeff Merkley of Oregon and Kamala Harris of California, as well as representatives Deb Haaland of New Mexico, David Cicilline of Rhode Island and Tom Malinowski of New Jersey.

“We urge you to promptly change your policy so that no more families endure its discriminatory effects,” the House Democrats said in their letter to Mr Pompeo, a former Tea Party Republican congressman who has expressed opposition to same-sex marriage but promised to defend gay rights around the world in his role as secretary of state.

The State Department, which has emphasised that the policy applies to opposite-sex and same-sex couples alike, declined to comment on Thursday.

The policy was first developed in the 1990s and is based on an interpretation of 1950s immigration law, which includes language that children are “born” of their parents and mentions a “blood relationship” in certain cases.

That interpretation has led the State Department to regard births from assisted reproductive technology as “out of wedlock” if the people providing the sperm and the egg are not married to each other.

Under the Obama administration, the State Department adjusted the requirement so that a parent could also establish a biological connection not only by supplying the egg or sperm but also by giving birth. That allows a lesbian couple to have a child “in wedlock” if one woman provides the egg and the other carries the baby. But two men in a marriage do not have that option.

“We find this to be deeply offensive,” the House Democrats said of the “out of wedlock” designation for children of same-sex couples.

While the policy predates Donald Trump’s election, the president’s critics have argued that the department’s efforts to preserve it are representative of other administration policies that have sought to dismantle protections for gay, bisexual and transgender people.

In their letter to Mr Pompeo, the senators highlighted the case of the Dvash-Banks family, a married Israeli-American gay couple who had twin sons in Canada using sperm from each of the fathers. The biological son of the American received citizenship, but his brother, the biological son of the Israeli, did not.

In February, a federal judge sided with the couple, calling the State Department’s interpretation of the immigration law “strained.” The department is appealing the decision.

The lawmakers called on the State Department to drop its appeal and “make it clear that every US married couple is entitled to the same rights under the US Constitution, no matter whom they love.”