(Reuters) — Twitter said on Thursday it had suspended hundreds of Russian-linked accounts and would ramp up enforcement of its spam rules as it probes online campaigns to influence the 2016 U.S. election.

Although the company’s disclosures in briefings to U.S. congressional staff and a public blog post were its most detailed to date on the issue, Senator Mark Warner, the top Democrat on the Senate Intelligence Committee, called the company’s statements “deeply disappointing.”

Warner, whose panel is investigating alleged Russian interference in the election, said Twitter officials had not answered many questions about the Russian use of the platform and that it was still subject to foreign manipulation.

Twitter has been criticized as being too lax in policing fake or abusive accounts.

Colin Crowell, Twitter’s vice president of public policy, was among company representatives who met behind closed doors with Senate Intelligence Committee aides on Thursday.

The company was also expected to brief the House of Representatives Intelligence Committee on Thursday, according to committee sources.

The intelligence committees on Wednesday asked executives from technology companies including Twitter, Facebook, Alphabet Inc’s Google to testify at a public hearing on Nov. 1 about alleged Russian interference.

The pressure on the companies reflects growing concern among lawmakers in both parties that social networks may have played a key role in Moscow’s attempts to spread disinformation and propaganda to sow political discord in the United States and help elect President Donald Trump. Moscow denies any such activity and Trump has denied any talk of collusion.

The San Francisco-based company said Russian media outlet RT, 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.

Low-quality tweets

Twitter said it would toughen restrictions on suspect spammers, for example by reducing the time that suspicious accounts stay visible during company investigations.

Twitter allows fictitious names and some automation by accounts, making it harder to distinguish improper activity.

Figures in the company’s blog showed the scale of the issue. In battling the automated promotion of trending topics, which get displayed to many users, the company said it counteracted 130,000 accounts daily.

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 work more closely with election and other officials and wanted to strengthen disclosures on optical 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, but said he was confident there would be bipartisan interest.