WASHINGTON — The brother of Saudi Crown Prince Mohammed bin Salman says he had "regular contact" with a missing Saudi journalist and critic before his disappearance last week, calling Jamal Khashoggi a "friend" as he dismissed allegations that Saudi Arabia killed or detained him "absolutely false, and baseless."

Prince Khalid bin Salman, who is also the Saudi Ambassador to Washington, said in a letter to "a few close contacts" Monday evening that the kingdom is taking "extraordinary" measures to find him "just as we would if it were any other Saudi citizen."

He downplayed the significance of Khashoggi’s "differences" with the kingdom's leaders, despite what he called Khashoggi’s "choice to go into his so called 'self-exile.'"

"Jamal has many friends in the Kingdom, including myself," Khalid wrote in the letter, which was obtained by NBC News.

The letter marks the most extensive defense to date by a top Saudi official since Khashoggi disappeared last Tuesday after visiting the Saudi Consulate in Istanbul, Turkey. It was not clear how many people received the note. NBC News obtained the letter independently and the Saudi Embassy in Washington confirmed its authenticity.

Khashoggi, though once close to the powerful inner circle of the Saudi royal family, had been in self-imposed exile since ambitious the crown prince became the kingdom’s putative next leader in June 2017.

In the letter, Khalid seeks to portray the relations between the kingdom and Khashoggi as normalized and amicable despite disagreements about views. He said as a Saudi citizen, Khashoggi’s "safety and security is a top priority for the Kingdom."

"This was not his first visit to the Consulate in Istanbul, as he regularly came to the Consulate (as well as the Embassy in Washington) in the last few months for citizen services," Khalid wrote.

Khalid stood behind the kingdom's claim that Khashoggi left the consulate on his own and that Saudi Arabia doesn’t know what happened to him after. Turkish authorities over the weekend said police believed he was killed in the consulate, and The Washington Post has reported Turkey believes he was killed by a Saudi hit team sent to Istanbul.

"I don't know who is behind these claims, or their intentions, nor do I care frankly," Khalid wrote.

On Monday, Turkish President Recep Tayyip Erdogan demanded that Saudi Arabia prove that Khashoggi truly left the consulate, questioning how consulate security cameras wouldn’t have caught it on tape. Saudi Arabia has maintained that its security cameras weren’t recording at the time.

Khashoggi disappeared last Tuesday after entering the Saudi consulate in Turkey’s largest city to pick up documents related to a divorce he needed before his upcoming marriage to a Turkish woman. He had come to the consulate several days earlier and been told to return to pick up the documents, three people with knowledge of Khashoggi's situation told NBC News.

The case has attracted growing international attention and demands for answers from governments around the world. If Saudi Arabia were confirmed to have been involved in his disappearance, it would mark an alarming escalation by the kingdom in its campaign, under the crown prince's leadership, to silence critics and opposition.

Yet the Trump administration, which has developed close ties to the crown prince Saudi Arabia, has walked a careful line since Khashoggi disappeared last week, avoiding direct criticism of the kingdom.

For the first week after the disappearance of Khashoggi, who wrote critical columns about Saudi Arabia in The Washington Post, no U.S. officials commented publicly on his whereabouts.

Even after Turkish authorities accused Saudi Arabia of murdering Khashoggi in the consulate, the lone reaction from the Trump administration was a one-sentence statement from an anonymous State Department official saying the U.S. couldn’t confirm the reports but was "closely following the situation."

On Monday, however, President Donald Trump broke his silence, expressing concern in response to a shouted question from a reporter as he returned from a trip to Florida.

"I am concerned about it," Trump said on the White House South Lawn. "I don't like hearing about it, and hopefully that will sort itself out right now nobody knows anything about it, but there's some pretty bad stories going around. I do not like it."

Shortly after, Vice President Mike Pence weighed in on Twitter, saying he was "deeply troubled" by the reports and saying that "if true, this is a tragic day."

"Violence against journalists across the globe is a threat to freedom of the press & human rights," Pence tweeted. "The free world deserves answers."

The vice president did not specify by name who he was calling on to provide those answers. Behind the scenes, though, top Trump administration aides have been reaching out to Saudi officials to seek information about Khashoggi’s whereabouts and the circumstances of his bizarre disappearance, administration officials told NBC News.

Later Monday, Secretary of State Mike Pompeo sent out a release saying, "We have seen conflicting reports on the safety and whereabouts of prominent Saudi journalist and Washington Post contributor Jamal Khashoggi. ‎As the President has conveyed, the United States is concerned by his disappearance. State Department senior officials have spoken with the Kingdom of Saudi Arabia through diplomatic channels about this matter. We call on the government of Saudi Arabia to support a thorough investigation of Mr. Khashoggi's disappearance and to be transparent about the results of that investigation."

It was the State Department's first official statement about the case.

"It is one of the hallmarks of this administration that they do not like to make any public criticism of their Arab authoritarian partners, and it seems Saudi Arabia is one of the most favored," said Amy Hawthorne of the Washington-based Project on Middle East Democracy."

Meanwhile, harsh reactions and demands for accountability are pouring in from U.S. lawmakers of both parties, as well as from other Western governments such as the United Kingdom. Sen. Lindsey Graham, a Republican who was Trump’s key Judiciary Committee ally in the fight over Supreme Court Justice Brett Kavanaugh, tweeted Monday morning that if the allegations of Saudi wrongdoing prove true, "there will be a heavy price to be paid — economically and otherwise."