US President Donald Trump has defended his "absolute right" to discuss sensitive material on terrorism and airline safety at a meeting with Russia's foreign minister.
US media reports said he had shared material that was passed on by a partner that had not given permission.
The White House refused to comment on reports that Israel was the source.
Though not illegal, Mr Trump's alleged gaffe is seen as a breach of trust by many in the intelligence community.
Mr Trump met Sergei Lavrov last week in the Oval Office, out of view of the US media.
Leading Republicans and Democrats have voiced concerns over what was said, with top Senate Democrat leader Chuck Schumer calling for the transcripts to be released by the White House.
The US Senate Intelligence Committee has also asked for copies of any notes taken in the meeting. CIA Director Mike Pompeo is due to brief the committee later.
Mr Trump's alleged disclosures are not illegal, as the US president has the authority to declassify information.
What was the president's defence?
Mr Trump tweeted: "As President I wanted to share with Russia (at an openly scheduled W.H. meeting) which I have the absolute right to do, facts pertaining to terrorism and airline flight safety.
"Humanitarian reasons, plus I want Russia to greatly step up their fight against [IS] & terrorism."
It is not clear if Mr Trump was acknowledging having shared intelligence secrets with the Russian officials, thus contradicting White House statements, or whether he was simply trying to explain what had been discussed.
The BBC's Anthony Zurcher in Washington says this was a carefully constructed defence of the meeting, in which President Trump frames any revelation of intelligence information as a calculated move to advance US national security priorities.
After all, the controversy that swirled around the White House on Monday night was never legal, it was political, and this defence may be enough for Republicans to rally around, he adds.
What happened in the Oval Office?
A report in the Washington Post on Monday said Mr Trump had confided top secret information relating to an IS plot thought to centre on the use of laptop computers on aircraft.
In a conversation with the Russian foreign minister and Russian ambassador Sergei Kislyak in the Oval Office on 10 May, the president revealed details that could lead to the exposure of a source of information, officials told the paper.
The intelligence disclosed came from a US ally and was considered too sensitive to share with other US allies, it added.
Others at the meeting realised the mistake and scrambled to "contain the damage" by informing the Central Intelligence Agency (CIA) and the National Security Agency (NSA), says the Post.
The meeting came a day after Mr Trump fired his FBI chief, James Comey, sparking criticism that he had done so because the FBI was investigating his election campaign's alleged Russian ties.
On Tuesday, US media reported the divulged material had been provided to the US by Israel, where Mr Trump is due to visit next week.
How has the White House responded?
Pressed by reporters on Tuesday, National Security Adviser HR McMaster declined to say whether or not Mr Trump had shared classified information with the Russians.
He denied the US president had caused a "lapse in national security".
"What the president discussed with the foreign minister was wholly appropriate to that conversation and is consistent with the routine sharing of information between the president and any leaders with whom he's engaged."
He also said President Trump had not been aware of the source of information that was discussed with the Russian officials.
Golden rule: Frank Gardner, BBC security correspondent
Despite the denials issued by the White House that any actual intelligence sources were revealed to the Russians, whatever was said in that Oval Office meeting was enough to alarm certain officials and, reportedly, to alert the CIA and NSA.
They in turn will have needed to warn the country that supplied the intelligence. There is a golden rule in the world of espionage that when one government supplies intelligence to another it must not be passed on to a third party without permission of the original supplier. The reason is simple: it could put the lives of their human informants at risk.
In this case it appears to relate to the discovery of plans by jihadists in Syria to devise a way of smuggling viable explosive devices on board a plane inside a laptop computer. Given the well-publicised ban on laptops in cabins on certain Middle Eastern routes, whoever revealed that information is unlikely to be still in place.
What has the reaction been?
- "This is dangerous and reckless" - Dick Durbin, Senate's second-highest ranked Democrat
- "Mr President, this isn't about your 'rights', but your responsibilities. You could jeopardise our sources, relationships and security" - Adam Schiff, top Democrat on House Intelligence Committee
- "A troubling signal to America's allies and partners around the world and may impair their willingness to share intelligence with us in the future" - Republican Senator John McCain
- "We have no way to know what was said, but protecting our nation's secrets is paramount" - spokesperson for Republican House Speaker Paul Ryan
- Congress could do with "a little less drama from the White House" - Mitch McConnell, Senate majority leader
- "We generally do not want to have anything to do with this nonsense" - Dmitry Peskov, Kremlin spokesman
- "If true, this is not going to instil confidence in allies already wary of sharing the most sensitive information" - senior Nato diplomat quoted by Reuters
Levels of US classification - from lowest to highest
- Confidential: Information that reasonably could be expected to cause damage to the national security if disclosed to unauthorised sources. Most military personnel have this level of clearance
- Secret: The same wording in the first sentence above, except it substitutes serious damage
- Top Secret: Again, the same wording except to substitute exceptionally grave damage
- Codeword: Adds a second level of clearance to Top Secret, so that only those cleared with the codeword can see it. Administered by the CIA. The material discussed by Mr Trump with the Russians was under a codeword, sources told the Washington Post.