New York (CNN) -- Syria, Iran and the militant group Hezbollah are undermining Lebanon's independence and are endangering its stability, the U.S. ambassador to the United Nations said Thursday.
Susan Rice spoke a day after three employees of the U.N.-backed court set up to try suspects in the 2005 murder of former Lebanese Prime Minister Rafik Hariri were attacked in Beirut.
"We continue to have deep concerns over Hezbollah's destructive and destabilizing influence in the region," said Rice, adding that Syria and Iran are largely responsible for the group's power.
Hezbollah is not accountable to Lebanon's democratic institutions, the diplomat said outside a U.N. Security Council meeting on Lebanon.
She singled out Syria for providing increasingly sophisticated weapons to militia groups, including Hezbollah.
Syria lashed back at Rice's accusations, saying she gave credibility to "wrong facts, wrong information."
Syria has not shipped arms into Lebanon, said Bashar Jaafari, the country's ambassador to the United Nations.
The U.S. government condemned the attack on the court employees.
Two investigators from the court's prosecutor's office and their interpreter were "violently attacked" by "a large group of people" who showed up at a Wednesday morning meeting that was part of the investigation, the United Nations said. The meeting was at a doctor's office.
The Lebanese army extracted the three staff members and brought them to the tribunal's office, where they received medical attention, the United Nations said.
The office of the prosecutor said several items were stolen from the staffers during the attack.
The tribunal is an independent body set up after an earlier U.N. mission found that Lebanon's own inquiry into the massive car bombing that killed Hariri and 22 others was seriously flawed and that Syria was primarily responsible for political tensions that preceded the bombing.
The latest attack "is yet another attempt to create a false choice between justice and stability and to prevent the independent tribunal from carrying out its Security Council mandate," State Department spokesman P.J. Crowley said in a statement issued Wednesday night.
"We again emphasize that efforts to discredit, hinder, or influence the tribunal's work serve only to increase instability and tensions in the country and should not be tolerated," he said.
The attack comes a day before a top leader from Hezbollah -- which the tribunal reportedly suspects of involvement in the killings -- will deliver a major speech challenging the court.
Hezbollah Secretary General Hassan Nasrallah is set to give a televised speech on the tribunal Thursday night.
On Wednesday, the court said it would not be deterred from trying the people it suspects killed Hariri and the others, calling the attack on its staffers "a deplorable attempt to obstruct justice."
"Those who carried out this attack must know that violence will not deter the Special Tribunal for Lebanon, a court of law, from fulfilling its mandate," the tribunal, based in The Hague, Netherlands, said in a news release.
Lebanese authorities have launched an investigation, the United Nations said.
Hariri's death stunned the nation and prompted tens of thousands to take to the streets, blaming Syria for the killing. Syria has always denied the accusations.
The explosion left a 10-foot crater in a street in downtown Beirut and had a similar effect on the country's political culture.
At the time of the killing, neighboring Syria had immense political influence in Lebanon.
A U.N. Security Council resolution demanded Damascus fully cooperate in the U.N. investigation. The assassination and subsequent investigation led to a complete withdrawal of Syrian troops from Lebanon after 30 years of military presence.
The tribunal's prosecutor, Daniel Bellemare, is expected to issue the long-awaited indictment by the end of the year.