Michele Geraci, an Italian economic development minister who is running the negotiations with Beijing, said in an interview that Chinese ships carrying materials from home or its vast network of interests in Africa through the Suez Canal simply needed to get their goods to central European markets as quickly as possible.

“Trieste meets that requirement swiftly,” he said.

Italian officials say their American counterparts initially seemed disinterested in the deal. Deputy Prime Minister Luigi Di Maio, the leader of the Five Star Movement, has made several trips to China in recent months, nearly signing the accord during a November visit to Beijing, they said. After the fact, American diplomats began making their case, but the Italians said the deal was noticeably not on the American radar during recent high-level meetings in Washington.

But this month, Garrett Marquis, spokesman for the American national security adviser, John R. Bolton, sharply attacked the deal in a Twitter post and in several interviews, while the National Security Council’s official Twitter account also issued a reproach on March 9.

“Endorsing BRI lends legitimacy to China’s predatory approach to investment and will bring no benefits to the Italian people,” the tweet stated, referring to the Belt and Road Initiative.

The Americans have also tried to pressure leaders of the nationalist League party, which is part of the governing coalition in Italy. This month, Trump administration officials and, separately, the former White House official Stephen K. Bannon, met with party leaders; Mr. Bannon said that he had warned his Italian allies in the League against what he called China’s “British East India Company model of predatory capitalism.”