MOSCOW — Russia’s top domestic security agency said Monday it has thwarted suicide bombings in Moscow planned by Islamic State operatives from Syria.

Four people have been arrested on suspicion of plotting attacks on the Moscow transit system and shopping malls, according to the Federal Security Service, or FSB.

Those arrested included two would-be suicide bombers along with an Islamic State envoy and an expert in explosives. One of them is a Russian national and three others are from former Soviet Central Asia, the FSB said.

The agency released a video in which its agents inspect a house used by the group to make explosives while two suspects lie down on the floor in handcuffs. It didn’t say when the arrests took place.

The FSB said the attacks were planned by two senior ISIS militants. The agency didn’t give their nationalities, but the names given by the FSB appear to indicate they hail from the former Soviet Union.

In May, the FSB arrested another group of suspected ISIS members who were also accused of plotting terror attacks in the capital.

The arrests follow a suicide bombing in St. Petersburg’s subway that left 16 dead and wounded more than 50 in April.

President Vladimir Putin said in April that some 9,000 militants, about half of them from Russia and the rest from former Soviet Central Asian nations, have joined the Islamic State in Syria.

He emphasized that a key goal for the Russian military operation in Syria is to crush them there and prevent them from coming back home.

In Syria on Monday, hundreds of rebels and civilians started leaving the Lebanon-Syria border area when a deal was reached for their departure after days of delay, the media arm of Lebanon’s militant Hezbollah group said.

In Beirut, meanwhile, the United States handed over to the Lebanese army eight Bradley Fighting Vehicles, part of a total of 32 that will be delivered over the coming months. The military aid is aimed at helping Lebanon combat extremist groups and prevent further spillover from neighboring Syria.

At the border, buses carrying members of the Levant People’s Brigades rebel group started moving from the Lebanese border town of Arsal in the direction of the Syrian village of Fleeta.

The evacuation comes nearly two weeks after more than 7,000 Syrians, many of them al-Qaida-linked fighters and their families, left Arsal following a Hezbollah offensive.

The Levant People’s Brigades, whose members did not take part in last month’s battles, will be heading to the Syrian town of Ruhaiba, about 30 miles northeast of the Syrian capital, Damascus, where they will return to normal life following an amnesty by the state, according to Hezbollah’s Al-Manar TV.

Rebels and government fighters have lived alongside each other in Ruhaiba without fighting for more than a year following a local de-escalation agreement.

Hezbollah’s Military Media said that by Monday afternoon, some 1,500 fighters and civilians had left toward Ruhaiba. Another 300 are scheduled to leave later in the day bound for their government-held hometowns in the western parts of the Qalamoun region, near the border with Lebanon.

The Britain-based Syrian Observatory for Human Rights said about 2,000 in total should enter Syria by the end of the day.

The only insurgents remaining on Lebanon’s side of the border now are members of the Islamic State group. Hundreds of ISIS fighters control a stretch of land that is almost equally split between Lebanon and Syria.

The Lebanese army has been preparing an attack for weeks, sending in reinforcements and pounding the area with artillery shells and rockets. The Syrian army and Hezbollah are preparing for a simultaneous attack on the Syrian side of the border.

At the Bradley handover ceremony at Beirut’s port, US Ambassador Elizabeth Richard said the vehicles represent an investment of more than $100 million that will ‘‘provide the Lebanese Armed Forces with new capabilities to protect Lebanon, to protect its borders and to fight terrorists.’’

The United States has supplied the Lebanese military with Cessna aircraft armed with Hellfire missiles, as well as helicopters, anti-tank missiles, artillery and radar, along with training.

The American Embassy says the United States has provided Lebanon with more than $1.4 billion in security assistance since 2005.