Amidst international scorn for Saudi Arabia’s official explanation of the death of Jamal Khashoggi, friends and colleagues of the reporter on Saturday resumed their vigil outside the Saudi consulate in Istanbul, where he was killed two weeks ago.

They were there to mourn his death and demand “true justice”.

In the US, Saudi Arabia’s major ally, the Trump administration remained cautiously supportive of Riyadh. But pressure was building. Bob Corker, chair of the powerful Senate foreign relations committee, doubted the Saudi version of events and said the US “must make its own independent, credible determination of responsibility for Khashoggi’s murder”.

After more than two weeks of stubborn denials from Saudi Arabia that it had anything to do with Khashoggi’s disappearance, statements carried on the state news agency in the early hours of Saturday acknowledged his death – the result, it said, of a “fistfight” inside the consulate on 2 October.

The statements said 18 men had been arrested in connection with the “cover-up” and two senior officials – confidantes of Crown Prince Mohammad Bin Salman – had been sacked. Khashoggi’s body was disposed of by a “local collaborator”, officials added.

Khashoggi was a Saudi national but a US permanent resident and a columnist for the Washington Post. Saudi Arabia had been under significant pressure from Washington to offer an explanation. Its eventual account met with widespread scepticism, however, in the face of mounting evidence from the Turkish investigation pointing to a sophisticated assassination operation that could not have been carried out without the sanction of the Saudi royal court.

Turkish officials have coordinated sustained leaks to local and US media that suggest Khashoggi was tortured and murdered and dismembered with a bone saw.

Investigators allege 15 men who arrived in Istanbul from Riyadh on jets owned by the Saudi royal family were sent to kill the reporter. Several of the suspects – including members of the crown prince’s security detail and a forensics expert – were captured in security camera footage entering the consulate building on the day Khashoggi died.

On Saturday, Saudi Arabia’s regional allies expressed support. The UK, another ally, cautiously acknowledged the Saudi version of events. But it was not clear the explanation of Khashoggi’s murder, the sackings and purported arrests would contain the damage to Riyadh.

At the United Nations, a spokesman said secretary general António Guterres was “deeply troubled” and “stresses the need for a prompt, thorough and transparent investigation … and full accountability for those responsible”. Outside government circles, Amnesty International called for “an impartial and independent investigation by the UN”.

In the US, the White House put out a statement “acknowledging” the Saudi account of “this tragic incident” and adding that it would follow developments.

In Nevada following a campaign rally, Donald Trump repeated the administration line that Saudi Arabia is too important a commercial and strategic partner

Bob Corker said: ‘We should not assume their latest story holds water.’ Photograph: Michael Reynolds/EPA

But it was becoming increasingly clear that Republican leaders in Congress were not going to give credibility to the Saudi version of events and would insist on a full investigation by US intelligence, potentially followed by sanctions mandated by legislation known as Global Magnitsky, after a murdered Russian lawyer.

“Saudi Arabia’s changing stories on Khashoggi’s murder is getting old,” the Florida senator Marco Rubio tweeted. “The latest one about a fistfight gone bad is bizarre. We must move forward with Global Magnitsky investigation we requested, find out what really happened and sanction those responsible.”

Corker said: “The story the Saudis have told about Jamal Khashoggi’s disappearance continues to change with each passing day, so we should not assume their latest story holds water. They can undergo their own investigation, but the US administration must make its own independent, credible determination of responsibility for Khashoggi’s murder under the Global Magnitsky investigation as required by law.”

The administration has 120 days from the day the Senate invoked Magnitsky, 10 October, to present the results of an investigation and a decision on sanctions.

In a statement, Washington Post publisher and chief executive Fred Ryan said Saudi Arabia had “shamefully and repeatedly offered one lie after another” and decried “an atrocity that appears to have been directed by the highest levels of their government”.

He added: “Offering no proof, and contrary to all available evidence, they now expect the world to believe that Jamal died in a fight following a discussion. This is not an explanation; it is a cover-up.”

At the consulate in Istanbul, TuranKışlakçı, president of the Arab Turkish Media Association, said in a speech: “We want Jamal’s murderers to be punished … and punishment also for the authority that gave the orders.”

The deputy head of Turkey’s ruling party, Numan Kurtulmuş, vowed that Turkey would “never allow a cover-up” of the killing. “We don’t immediately blame anyone. But we won’t go along with leaving details buried,” he said.

Separately, a senior Turkish official told Reuters investigators were close to finding out what happened to Khashoggi’s body. Police were searching Belgrad forest, north of Istanbul, and farmland near Yalova, 55-mile drive south of the city, after using CCTV footage to track two vehicles owned by the Saudi consulate after Khashoggi was killed.

Khashoggi’s fiancée, Hatice Cengiz, wrote in a tweet: “The heart grieves, the eye tears, and with your separation we are saddened, my dear Jamal.”

Tawakkol Karman, a Yemeni Nobel peace prize laureate, told the Guardian: “Personally I have lost a friend and adviser. But the world has lost an important voice and the Arab Spring one of its most important defenders.

“Saudi Arabia wanted to silence him forever but instead he has become an international icon for freedom of expression. This crime cannot go unpunished. Spilling his blood has only made the case stronger against the cruelty and repression of Saudi Arabia.”