WASHINGTON – Boston officials concerned about potential terror strikes are asking federal regulators to block a proposed natural gas pipeline until they get assurances the project won’t bring more tanker traffic to the city’s harbor.

“Any project that would mean additional LNG ship traffic in Boston Harbor, we would be adamantly opposed to,” James W. Hunt, the city’s chief of environment and energy, said Thursday.

Suez-Distrigas, which operates the liquefied natural gas terminal in nearby Everett, Mass., and its partner, Tennessee Gas Pipeline Co., are seeking approval from the Federal Energy Regulatory Commission for a new 24-inch, 7.8-mile pipeline in Middlesex and Essex counties.

The LNG terminal is on the Mystic River, just north of Boston. To reach it, tankers must pass within a mile of downtown Boston and under the landmark Tobin Bridge. Local officials have long warned that a terror strike on a tanker crossing Boston Harbor could be catastrophic.

Suez-Distrigas officials say they don’t expect an increase in tanker traffic, but city officials want stronger assurances.

“The FERC should not license the proposed project until the relationship between the expansion of pipeline capacity and the LNG ship traffic to the Everett LNG facility is clearly understood,” the city wrote in a letter sent Wednesday to the federal agency.

Boston officials also called for a federal cap on LNG tanker shipments to Everett should the new pipeline be approved.

“The city’s prime concern, voiced frequently by Mayor Thomas M. Menino since Sept. 11, 2001, is the need to reduce and ultimately cease all LNG shipments through Boston Harbor,” city officials wrote.

Suez-Distrigas said it did not expect LNG tanker deliveries to Everett to increase because the new pipeline was replacing “other” pipeline capacity, which it declined to identify.

“We don’t anticipate a change in the range of deliveries to Everett,” said Suez-Distrigas spokeswoman Julie Vitek.

Menino, who said he hopes to balance the region’s energy needs with security concerns, sued to ban LNG tankers from the harbor after the Sept. 11 attacks. Deliveries to the Everett terminal were halted temporarily, but federal homeland security officials eventually allowed them to resume.