The UK's foreign secretary has told Saudi Arabia that Britain expects urgent answers over the disappearance of Saudi journalist Jamal Khashoggi.
In a phone call to Saudi Arabian Foreign Minister Adel al-Jubeir, Jeremy Hunt warned that "friendships depend on shared values".
Mr Khashoggi was last seen visiting the Saudi consulate in Istanbul last week, and Turkey says he may have been murdered there.
Saudi Arabia denies the suggestion.
Just met the Saudi ambassador to seek urgent answers over Jamal Khashoggi. Violence against journalists worldwide is going up & is a grave threat to freedom of expression. If media reports prove correct, we will treat the incident seriously - friendships depend on shared values.— Jeremy Hunt (@Jeremy_Hunt) October 9, 2018
Taking a tougher line than the Trump administration, a spokesperson for the UK's Foreign Office said, if media reports surrounding the case were correct, the UK would treat the incident "very seriously."
Earlier on Tuesday, US President Donald Trump said he had not yet spoken to Saudi officials about the journalist's disappearance.
"I have not. But I will be at some point," he told reporters. "I know nothing right now. I know what everybody else knows - nothing."
In a separate development, Mr Khashoggi's Turkish fiancée Hatice Cengiz appealed to the US for help.
In an emotional article in the Washington Post, she wrote: "I implore President Trump and first lady Melania Trump to help shed light on Jamal's disappearance."
"We were in the middle of making wedding plans, life plans," when he vanished, she said.
"Jamal is a valuable person, an exemplary thinker and a courageous man who has been fighting for his principles. I don't know how I can keep living if he was abducted or killed in Turkey."
Turkey says it will conduct a search of Saudi Arabia's consulate in Istanbul.
Saudi Arabia's foreign ministry said the country was "open to co-operation" and a search of the building could go ahead as part of the investigation.
It says the journalist left the consulate shortly after arriving, while Turkey says he was not seen leaving the building.
Ankara is demanding that Saudi Arabia prove he left, while not providing evidence to support the claim he was killed inside.
Who is Jamal Khashoggi?
A critic of Crown Prince Mohammed bin Salman, Mr Khashoggi was living in self-imposed exile in the US and writing opinion pieces for the Washington Post before his disappearance.
A former editor of the al-Watan newspaper and a short-lived Saudi TV news channel, he was for years seen as close to the Saudi royal family. He served as an adviser to senior Saudi officials.
But after several of his friends were arrested, his column was cancelled by the al-Hayat newspaper and he was allegedly warned to stop tweeting, Mr Khashoggi left Saudi Arabia for the US.
What's been the reaction to the disappearance?
US Secretary of State Mike Pompeo called on Saudi Arabia to "support a thorough investigation" of his disappearance and "to be transparent about the results".
UN experts have demanded a "prompt independent and international investigation" into his disappearance.
Last week, Crown Prince Mohammed told Bloomberg News that his government was "very keen to know what happened to him", and that Mr Khashoggi had left "after a few minutes or one hour".
An unacceptable line?
Analysis by BBC diplomatic correspondent James Robbins
The UK has apparently told the Saudis they need to show Mr Khashoggi is alive, and the best way to do that would be for him to appear on television.
If that doesn't happen soon, it's clear that Britain and its allies are likely to conclude that Saudi Arabia has crossed an unacceptable line.
In the wake of the Salisbury attack, and the international punishment meted out to Russia, it would then be increasingly difficult for Western governments to avoid action against the Saudi kingdom, however close their military and economic ties.
Crown Prince Mohammed's brother and the Saudi ambassador to the US, Prince Khaled bin Salman al-Saud, has insisted all the reports about his disappearance or death "are completely false and baseless".
"Jamal has many friends in Saudi Arabia, and I am one of them," he said in a statement, saying the two had kept in touch while he was living abroad "despite differences".
When was he last seen?
Mr Khashoggi went to the consulate last Tuesday to obtain a document certifying he had divorced his ex-wife, so that he could remarry Hatice Cengiz.
She said Mr Khashoggi had been required to surrender his mobile phone, which is standard practice in some diplomatic missions.
He told her to call an adviser to Turkish President Recep Tayyip Erdogan if he did not return. She eventually called the police.
What have both countries said?
Turkish officials say Mr Khashoggi was killed on the premises and his body was then removed.
"We believe that the murder was premeditated and the body was subsequently moved out of the consulate," an unnamed source told Reuters.
A source quoted by The Washington Post said the journalist was killed by a 15-member Saudi team sent "specifically for the murder".
The head of the Turkish-Arab Media Association, Turan Kislakci, told the New York Times that Turkish police officers providing security for the consulate had checked their security cameras, which did not show the journalist leaving on foot.
Turkey reportedly says it is scouring road cameras for a black van believed to have carried Mr Khashoggi's body from the consulate, while the Washington Post released CCTV footage of the journalist entering the building on Monday.
Saudi Arabia says the allegations are baseless. It has allowed journalists into the consulate to show Mr Khashoggi is not there, reportedly even opening cupboards.
The BBC's Security Correspondent Frank Gardner says one possible scenario is that he was abducted inside the consulate, driven out in a diplomatic vehicle and "rendered" back to Saudi Arabia to either face retribution or be held incommunicado under indefinite house arrest.
BBC Newshour interviewed the journalist just three days before his visit to the consulate, and in an off-air conversation asked if he would ever return to his home country.
"I don't think I'll be able to go home," he told the BBC, saying that in Saudi Arabia "the people who are arrested are not even dissidents" and saying he wished he had a platform at home to write and speak freely at this time of "great transformation" in his country.
Three days before he disappeared, we interviewed Jamal Khashoggi. Off air, we asked him about the possibility of returning to Saudi Arabia. We wouldn't normally release this conversation but we've decided to make an exception in light of the circumstances. https://t.co/Qj9pQ2ExbP pic.twitter.com/cmWimMj93H— BBC Newshour (@BBCNewshour) October 8, 2018
The programme has released audio of the conversation, saying that although it would not normally do so, it had decided to make an exception "in light of the circumstances".