CAIRO — After days of anti-American turmoil, governments in the Muslim world are striving to soothe the anger of their citizens while convincing the United States of their goodwill. But US diplomatic outposts remained under threat Sunday, and the Hezbollah militant group called for more protests.

In Pakistan, at least one protester was killed and 18 were injured Sunday as hundreds of people broke through a barricade in a march to the US Consulate in Karachi, and thousands more rallied in Lahore, where American flags were burned.

In a move that could escalate tensions around the Arab world, Hezbollah leader Sheik Hassan Nasrallah said in a televised speech that the US government must be held accountable for an anti-Muslim film that was produced in the United States.

‘‘The ones who should be held accountable and boycotted are those who support and protect the producers, namely the US administration,’’ Nasrallah said. He called for protests on Monday, Wednesday, Friday, Saturday, and Sunday.

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Susan Rice, the US ambassador to the United Nations, said Sunday that last week’s deadly assault on the US consulate in Libya was a spontaneous reaction to the anti-Muslim video, but Libya’s interim president said the attackers spent months preparing and carefully choosing their date — the anniversary of the Sept. 11 terrorist attacks.

Unnerved by the attack Tuesday that claimed the life of the US ambassador to Libya and three other Americans, the Obama administration last week launched an investigation into whether terrorist groups had exploited outrage over the video to trigger an attack long in the works.

Rice said evidence shows no indication of a premeditated or coordinated strike. She said the attack in Benghazi, powered by mortars and rocket-propelled grenades, appeared to be a copycat of demonstrations that had erupted hours earlier outside the US Embassy in Cairo. The attacks were spurred by a YouTube film attributed to a California man that mocked the prophet Mohammed. Washington has condemned the film.

‘‘It seems to have been hijacked, let us say, by some individual clusters of extremists who came with heavier weapons,’’ Rice said in interviews on Sunday news shows. Whether those extremists had ties to Al Qaeda or other terrorist groups has yet to be determined, Rice said, noting that the FBI is still investigating.

Libya’s interim president, Mohammed el-Megarif, said there was no doubt the perpetrators had predetermined the date of the attack. ‘‘It was planned, definitely. It was planned by foreigners, by people who entered the country a few months ago,’’ he said.

Megarif, who appeared on CBS’s “Face the Nation’’ on Sunday, also announced the arrests of about 50 people in connection with the Benghazi consulate attack.

Representative Mike Rogers, Republican of Michigan and chairman of the House Intelligence Committee, said it was premature to rule out a premeditated attack. A former FBI agent, Rogers said there were too many coincidences to conclude the Benghazi attack had not been planned.

Senator John McCain of Arizona, the top Republican on the Senate Armed Services Committee, said, ‘‘Most people don’t bring rocket-propelled grenades and heavy weapons to a demonstration.’’

In the Pakistan attack, police fired tear gas and water cannons at the protesters in Karachi after they broke through the barricade and reached the outer wall of the US Consulate, police officer Mohammad Ranjha said. The protesters threw stones and bricks, prompting the police to beat back the crowd with batons. The police and private security guards also fired in the air to disperse the crowd.

The demonstration in Lahore was organized by Jamaat-ud-Dawa, believed to be a front organization for the militant group blamed for attacks in Mumbai, India, in 2008 that killed more than 160 people. The protesters shouted anti-US slogans and burned an American flag.

In Cairo, the US Embassy returned to full staffing Sunday, a spokesman said, for the first time since Tuesday’s protests.

But the American diplomatic presence remained reduced elsewhere in the region, meaning that there were fewer routes to repair relations as they came under the most strain since last year’s Arab Spring uprisings.

In Tunisia, where additional security has been deployed to protect the embassy, the Saturday decision to withdraw nonessential US Embassy staff from the mission there appeared to jar Tunisian officials, who have marketed the country as a model of democratic transformation after the toppling of president Zine El Abidine Ben Ali.

Tunisia’s 2011 protests led to newfound freedoms for many citizens to express their distaste for their own governments and for the United States. Leaders have struggled ever since to accommodate those anti-American sentiments.

In an address to the nation Friday night, President Moncef Marzouki of Tunisia condemned that day’s attack on the US Embassy and an American school, in which four protesters were killed.

He said those who organized the protest had ‘‘crossed a red line.’’ Yet he also sought to appease the sentiments of those angered by the film, saying Tunisia would work with Egypt to sue its producers.

Hedi Ben Abbas, a foreign affairs minister of state whose portfolio includes the Americas, said in an interview Sunday that the Tunisian government ‘‘deeply regrets’’ the American decision to pull its diplomats after the attack.

Security officials, Ben Abbas said, were overhauling their procedures and could now ‘‘guarantee’’ the safety of all diplomatic facilities and foreigners. He said the government, which is headed by a moderate Islamist party that has faced criticism for tolerating religious zealotry, is determined to respond more firmly than it has to previous violent protests staged by Salafists. Dozens of people suspected of involvement in the riot have been arrested, according to local news reports.

Friday’s demonstration, Ben Abbas said, ‘‘was for us the end of the game.’’

‘‘The government of America cannot be responsible for the movie,’’ he said. Similarly, he said, ‘‘The Americans cannot blame the Tunisian government for the behavior’’ of protesters.

‘‘The United States should trust us again,’’ Ben Abbas added. ‘‘We need them more than ever to support democracy.’’

Egypt and the Obama administration had been working out the final points of an aid deal that could forgive a sizeable chunk of the more than $3 billion that the Egyptian government owes to the United States. But the final outcome of the aid talks remains uncertain, as does congressional support for a deal.

On Sunday, Morsi met in Cairo with President Omar al-Bashir of Sudan, despite an International Criminal Court warrant for the Sudanese leader’s arrest — and despite a sharp decline in relations between Sudan and the United States since Friday, when the US Embassy was attacked by protesters in Khartoum.