WASHINGTON -- A Russian woman has pleaded not guilty in a U.S. federal court to charges that she violated the U.S. foreign-agent law as part of a Russian effort to infiltrate the U.S. political system.

Judge Deborah Robinson ordered the 29-year-old woman, Maria Butina, to be held without bail until a trial is held, citing U.S. prosecutors' arguments that the risk she might flee the United States was too great. No date has been set for the trial.

During the July 18 hearing at U.S. District Court, prosecutors also disclosed that a separate fraud investigation is being conducted into one of two U.S. citizens referred to in the complaint against Butina.

Butina, who recently received a graduate degree from American University, was engaged in a "covert influence campaign," the lead U.S. prosecutor, Erik Kenerson, told the court.

"We do not believe she was here...just to attend American University," he said.

Defense lawyer Robert Driscoll entered the not-guilty plea during a two-hour hearing, in which U.S. officials laid out some of the evidence for the charges, which included conspiracy and failure to register under the Foreign Agents Registration Act, a decades-old law passed originally to curb Nazi propaganda.

Driscoll denied Butina was a Russian agent and that she was a flight risk.

Dressed in a standard orange prison jumpsuit, Butina said nothing during the hearing and had no reaction when the judge refused pleas for her to be released pending trial.

The hearing came a day after U.S. prosecutors filed the additional conspiracy charge against her, raising the possibility that others might face connected criminal charges.

Moscow on July 18 condemned Butina’s arrest and claimed it was aimed at undermining the summit between Presidents Donald Trump and Vladimir Putin in Helsinki.

At a briefing in Moscow before Butina's court hearing, Foreign Ministry spokeswoman Maria Zakharova asserted that the charges were baseless and stemmed from "Russophobic hysteria" in Washington.

"You have the impression that someone must have grabbed a watch and a calculator to determine when the decision on Butina's arrest should be made in order to undermine the outcomes of the summit," she said. "It was deliberately timed."

In court filings, U.S. prosecutors accused Butina of cultivating ties with U.S. politicians and political organizations at the direction of a Russian lawmaker and government official identified as a deputy chairman of the Russian Central Bank.

Court records did not name the official, but the description matches that of Aleksandr Torshin, a member of the ruling United Russia political party. Butina has appeared in numerous photographs on her Facebook page with Torshin, who was sanctioned by the U.S. Treasury Department.

Torshin is also under investigation in Spain as part of a wide-reaching probe into Russian organized crime there.

In an interview with Russia's state-owned Interfax news agency, Andrei Kolyadin, a Kremlin official and former member of the presidential administration, said he hired Butina as an interpreter last year when he attended a National Prayer Breakfast in Washington.

The event attracted leading U.S. conservative politicians.

The Justice Department said in its complaint that Butina worked with two unnamed U.S. citizens and her boss in Moscow to try to influence American politics and infiltrate the National Rifle Association, a powerful U.S. gun rights organization, to establish "back-channel" lines of communication with U.S. politicians.

Nikolai Kovalyov, a State Duma lawmaker who used to head Russia's main intelligence agency, the Federal Security Service, said authorities announced the charges during Trump's summit with Putin on July 16 to undermine the leaders' efforts to improve Russian-U.S. ties.

The U.S. charges were filed under seal on July 14, and Butina was detained the following day. The charges were announced publicly on July 16, following the conclusion of the Helsinki summit.

Kovalyov was quoted by Russian state-run media as saying that U.S. security agencies are "working against their own president by interfering in politics."

Mikhail Fedotov, chairman of the Russian Presidential Human Rights Council, also was quoted by Interfax as saying that Butina's arrest was intended to undermine the Putin-Trump summit and sets a "bad example."

With reporting by Reuters, Interfax, and TASS