Biden meets with top cyber officials after the latest attack, likely by Russians.

President Biden is gathering top advisers in the Situation Room on Wednesday to develop a strategy to counter increasingly brazen ransomware attacks by Russia-based hackers.

The meeting comes as several recent attacks test the red lines set by President Biden during his high-stakes summit with President Vladimir V. Putin of Russia last month.

The White House did not provide a list of attendees to Wednesday’s meeting, which was planned for 9:30 a.m., but the key players are from agencies including the Department of Justice and the Department of Homeland Security.

On Sunday, a Russia-based cybercriminal organization known as REvil claimed responsibility for a cyberattack over the long holiday weekend that has spread to 800 to 1,500 businesses around the world. It was one of the largest attacks in history in which hackers shut down systems until a ransom is paid, security researchers said.

Just days later, the Republican National Committee said Tuesday that one of its technology providers, Synnex, had been hacked. While the extent of the attempted breach remained unclear, the committee said none of its data had been accessed.

Early indications were that the culprit was Russia’s S.V.R. intelligence agency, according to investigators in the case. The S.V.R. is the group that initially hacked the Democratic National Committee six years ago and more recently conducted the SolarWinds attack that penetrated more than a half-dozen government agencies and many of the largest U.S. corporations.

It was unclear whether the REvil and R.N.C. attacks were related. But they are a test for Mr. Biden just three weeks after he held his first meeting as president with Mr. Putin, one in which he demanded that the Russian leader rein in ransomware activities against the United States. At the meeting, Mr. Biden said later, he presented Mr. Putin with a list of 16 critical sectors of the American economy that, if attacked, would provoke a response — though he was cagey about what that response would be.

The newest attacks appeared to cross many lines that Mr. Biden has said he would no longer tolerate. On the campaign trail last year, he put Russia “on notice” that, as president, he would respond aggressively to counter any interference in American elections. Then in April, he called Mr. Putin to warn him about impending economic sanctions in response to the SolarWinds breach.

The likely S.V.R. breach of Synnex left unclear whether the R.N.C. was the target or whether it was unintended collateral damage in a broader hack that may not have been directed at the Republicans.