WASHINGTON/SAN FRANCISCO (Reuters) - Twitter TWTR.N said on Thursday it had suspended about 200 Russian-linked accounts as it probes online efforts to meddle with the 2016 U.S. election, but an influential Democratic senator slammed its steps as insufficient.

Senator Mark Warner, the top Democrat on the Senate Intelligence Committee, summoned Twitter officials to testify behind closed doors on Thursday as part of broad investigation of Russian influence in the 2016 presidential election. Facebook FB.O faced a similar grilling earlier this month.

Lawmakers in both parties suspect social networks may have played a big role in Moscow’s attempts to spread propaganda, sow political discord in the United States and help elect President Donald Trump. Moscow denies any such activity, and Trump has denied any collusion.

Twitter also briefed the House of Representatives Intelligence Committee on Thursday.

Warner said Twitter officials had not answered many questions about Russian use of the platform and that it was still subject to foreign manipulation.

The company’s presentation to the Intelligence Committee “showed an enormous lack of understanding from the Twitter team of how serious this issue is,” Warner said. He took particular umbrage at what he said was Twitter’s decision to largely confine its review to accounts linked to fake profiles already spotted by Facebook.

Twitter said it had identified and removed 22 accounts directly linked to about 500 fake Facebook pages or profiles tied to Russia and that it unearthed an additional 179 accounts that were otherwise related.

Twitter declined to comment when asked about Warner’s comments.

In addition to the private testimony by its officials, the company published a public blog post Thursday with its most detailed discussion to date of the steps it was taking to combat propaganda.

Warner in remarks to reporters called Twitter’s statements “deeply disappointing” and “inadequate on almost every level.”

The comments signaled that the congressional investigations into Russia's use of social media platforms would not ease up. Twitter, Facebook and other Internet companies including Alphabet Inc's Google GOOGL.O are facing a steady stream of criticism as more information emerges about manipulation of their platforms during the 2016 election campaign.

Users, lawmakers and technology analysts have long criticized Twitter as too lax in policing fake or abusive accounts. Unlike Facebook, Twitter allows both anonymous accounts and automated accounts, or bots, making it far more difficult to police the service.

On Thursday, researchers at Oxford University published a study concluding that Twitter bots disseminated misinformation and propaganda at a higher rate in U.S. battleground states than in noncompetitive states during a 10-day period around Election Day in November.

San Francisco-based Twitter said Russian media outlet Russia Today, which is close to the Kremlin, had spent $274,100 on Twitter advertisements and promoted 1,823 tweets potentially aimed at the U.S. market.

Those ad buys alone topped the $100,000 that Facebook this month linked to a Russian propaganda operation during the 2016 election cycle, a revelation that prompted calls from some Democrats for new disclosure rules for online political ads.

Representative Adam Schiff, the top Democrat on the House Intelligence Committee, was more tempered in his assessment of Twitter’s briefing, saying in a statement that the firm expressed a desire to work cooperatively with investigators and conduct additional analyses.

‘LOW-QUALITY TWEETS’

The Twitter logo is displayed on a screen on the floor of the New York Stock Exchange (NYSE) in New York City, U.S., September 28, 2016. REUTERS/Brendan McDermid/File Photo

Twitter announced new measures to toughen restrictions on suspect spammers, for example by reducing the time that suspicious accounts stay visible during company investigations.

To thwart abuse via applications interacting with Twitter, the company said it had suspended 117,000 apps since June that had been responsible for 1.5 billion “low-quality” tweets this year.

Twitter said it wanted to strengthen disclosure rules on political advertising, as Facebook has just done.

Warner is leading efforts to introduce legislation requiring internet platforms to reveal who is purchasing online political ads, which would bring them in line with rules governing ads on radio or television.

He told reporters on Thursday he did not have a Republican co-sponsor for a draft measure he was circulating he was confident there would be bipartisan interest.