A hacking group linked to the North Korean regime is accused of stealing hundreds of millions of dollars by infiltrating the computer systems of banks in at least 11 countries since 2014.

A report by the American cybersecurity firm FireEye described the group -- which it dubbed APT38 -- as "a large, prolific operation with extensive resources" and warned that it "remains active and dangerous to financial institutions worldwide."

The document says APT38's initial operations targeted banks in Asian countries -- including Vietnam, Malaysia and Taiwan -- where North Korea had experience in money laundering, but then expanded into such far-flung countries as Brazil, Turkey, Mexico and the United States.

In all, FireEye says, APT38 has attempted to steal $1.1 billion, and based on the data it can confirm, has gotten away with hundreds of millions in dollars. It has used malware to insert fraudulent transactions in the Society for Worldwide Interbank Financial Telecommunication or SWIFT system that is used to transfer money between banks. Its biggest heist to date was $81 million stolen from the central bank of Bangladesh in February 2016. The funds were wired to bank accounts established with fake identities in the Philippines. After the funds were withdrawn they were suspected to have been laundered in casinos.

The FireEye report was made public one day after the U.S. Department of Homeland Security warned of the use of malware by Hidden Cobra, the federal government's byword for North Korea hackers, in fraudulent ATM cash withdrawals from banks in Asia and Africa. It said that Hidden Cobra was behind the theft of tens of millions of dollars from teller machines in the past two years. In one incident this year, cash had been simultaneously withdrawn from ATMs in 23 different countries, it said.

North Korea, which prohibits access to the world wide web for virtually all its people, has previously denied involvement in cyberattacks, and attribution for such attacks is rarely made with absolute certainty. It is typically based on technical indicators such as the Internet Protocol, or IP, addresses that identify computers and characteristics of the coding used in malware, which is the software a hacker may use to damage or disable computers.

This activity has been taking place against the backdrop of a dramatic diplomatic shift as Kim Jong Un has opened up to the world. He has held summits with South Korean President Moon Jae In and with President Trump, who hopes to persuade Kim to relinquish the nuclear weapons that pose a potential threat to the U.S. homeland. Tensions on the divided Korean Peninsula have dropped and fears of war with the U.S. have ebbed. Trump this weekend will dispatch his top diplomat, Mike Pompeo, to Pyongyang for the fourth time this year to make progress on denuclearization.

But North Korea has yet to take concrete steps to give up its nuclear arsenal, so there's been no let-up in sanctions that have been imposed to deprive it of fuel and revenue for its weapons programs, and to block it from bulk cash transfers and accessing the international banking system.

Sandra Joyce, FireEye's head of global intelligence, said that while APT38 is a criminal operation, it leverages the skills and technology of a state-backed espionage campaign, allowing it to infiltrate multiple banks at once and figure out how to extract funds. On average, it dwells in a bank's computer network for 155 days to learn about its systems before it tries to steal anything. And when it finally pounces, it uses aggressive malware to wreak havoc and cover its tracks.

"We see this as a consistent effort, before, during and after any diplomatic efforts by the United States and the international community," said Joyce, describing North Korea as being "undeterred" and urging the U.S. government to provide more specific threat information to financial institutions about APT38's modus operandi. APT stands for Advanced Persistent Threat.

The Silicon Valley-based company says it is aware of continuing, suspected APT38 operations against other banks. The most recent attack it is publicly attributing to APT38 was against one of Chile's biggest commercial banks, Banco de Chile, in May this year. The bank has said a hacking operation robbed it of $10 million.

FireEye, which is staffed with a roster of former military and law-enforcement cyberexperts, conducted malware analysis for a criminal indictment by the Justice Department last month against Park Jin Hyok, the first time a hacker said to be from North Korea has faced U.S. criminal charges. He's accused of conspiring in a number of devastating cyberattacks: the Bangladesh heist and other attempts to steal more than $1 billion from financial institutions around the world; the 2014 breach of Sony Pictures Entertainment; and the WannaCry ransomware virus that in 2017 infected computers in 150 countries.

The Associated Press contributed to this report.