Facebook Inc is much better than it was in 2016 at tackling election interference but cannot guarantee the site will not be used to undermine European Parliament elections in May, Chief Executive Officer Mark Zuckerberg said on Tuesday.

Chastened since suspected Russian operatives used Facebook and other social media to influence an election that surprisingly brought Donald Trump to power in the United States, Facebook has said it has ploughed resources and staff into safeguarding the May 26 EU vote.

Zuckerberg said there had been a lot of important elections since 2016 that have been relatively clean and demonstrated the defenses it has built up to protect their integrity.

“We’ve certainly made a lot of progress ... But no, I don’t think anyone can guarantee in a world where you have nation states that are trying to interfere in elections, there’s no single thing we can do and say okay we’ve now solved the issue,” Zuckerberg told Irish national broadcaster RTE in an interview.

“This is an ongoing arms race where we’re constantly building up our defenses and these sophisticated governments are also evolving their tactics.”

U.S. intelligence agencies concluded that Russia ran a disinformation and hacking operation to undermine the American democratic process and help Republican Trump’s 2016 campaign. Moscow denies interfering in the election.

Under pressure from EU regulators to do more to guard against foreign meddling in the bloc’s upcoming legislative election, Facebook toughened its rules on political advertising in Europe last week.

It also announced plans to ramp up efforts to fight misinformation ahead of the vote and will partner with German news agency DPA to boost its fact checking.