WASHINGTON (AP) ― The U.S. objected Wednesday to Iran’s anticipated selection of a former hostage-taker at the American Embassy in Tehran as its newest ambassador to the United Nations. But the Obama administration stopped short of saying it would refuse him a visa to enter the U.S.

State Department deputy spokeswoman Marie Harf called the potential nomination of Hamid Aboutalebi “extremely troubling” and said the U.S. has raised its concerns with Tehran. Lawmakers in Congress who usually disagree on everything ― ranging from liberal New York Democrat Sen. Chuck Schumer and conservative Texas Republican Sen. Ted Cruz ―have demanded that Aboutalebi be barred from living and working in the United States.

But Harf noted that except in limited cases, the U.S. is generally obligated to admit the chosen representatives of member states to U.N. headquarters in New York.

“We’re taking a close look at the case now, and we’ve raised our serious concerns about this possible nomination with the government of Iran,” Harf told reporters. She added: “But we do take our obligations as host nation for the United Nations very seriously.”

Hamid Babaei, a spokesman for Iran’s Mission to the United Nations, said Wednesday, “It has been a usual practice in the Iranian Foreign Ministry to formally announce and appoint ambassadors ― to all foreign postings ― once all the formalities are completed.”

Aboutalebi was a member of a Muslim student group that stormed the U.S. Embassy in Tehran in 1979 and held 52 Americans hostage for 444 days. He reportedly has insisted that his involvement in the group -- Muslim Students Following the Imam‘s Line -- was limited to translation and negotiation.

But lawmakers have derided Aboutalebi as a terrorist and a key conspirator in the hostage crisis, and warned that his nomination would escalate tensions in already delicate negotiations aimed at curbing Tehran’s nuclear program.

In a letter Wednesday to Secretary of State John Kerry, Schumer said Aboutalebi‘s association with the student group should make him ineligible for a visa or diplomatic immunity in the U.S.

“This man has no place in the diplomatic process,” Schumer said in a separate statement. “Iran’s attempt to appoint Mr. Aboutalebi is a slap in the face to the Americans that were abducted, and their families. It reveals a disdain for the diplomatic process and we should push back in kind.”

Earlier this week, Cruz questioned the wisdom of negotiating with the Iranian government in light of what he called its “deliberately insulting and contemptuous” choice for U.N. ambassador. The nuclear negotiations are set to resume next week in Vienna.

Harf insisted the nuclear talks and the U.N. envoy were separate issues, even as she noted Washington‘s ongoing concerns with Iran on multiple fronts.

“On the nuclear issue, they have upheld their commitments,” Harf said. “We hope they will continue to.”