Danish and Norwegian naval vessels are to head for waters off Syria again Friday in readiness to escort a delayed shipment of key chemical weapons for destruction, a spokesman said.

The ships will weigh anchor from the nearby island of Cyprus, where they headed to port on Monday when it became clear that a year-end deadline for the shipment would be missed, Norwegian armed forces spokesman Lars Hovtun said.

Russian President Vladimir Putin on Tuesday threatened "terrorists" with total destruction after twin suicide strikes claimed 34 lives and raised alarm over security at the Sochi Winter Olympic Games.

A bombing at the main railway station of the southern city of Volgograd killed 18 people on Sunday while a second strike that hit a trolleybus on Monday claimed 16 lives.

Russia beefed up security and mourned its dead on Tuesday as the toll from jarring successive-day suicide strikes in the run-up to the Sochi Winter Olympic Games rose to 34.

The southern city of Volgograd planned to bury the first victims of Sunday's bombing of the heartland hub's main railway station that killed 18 people and which officials have provisionally blamed on a young woman from the restive North Caucasus region of Dagestan.

The United States on Monday called for closer security cooperation with Russia ahead of the Sochi Winter Olympics, after two deadly bombings in the city of Volgograd.

"The U.S. government has offered our full support to the Russian government in security preparations for the Sochi Olympic Games, and we would welcome the opportunity for closer cooperation for the safety of the athletes, spectators, and other participants," National Security Council spokeswoman Caitlin Hayden said in a statement.

Switzerland said Monday it had granted a three-month visa to Kremlin critic Mikhail Khodorkovsky, who has been staying in a German luxury hotel since his release from a Russian prison.

Khodorkovsky, a former oil tycoon, was jailed for 10 years for financial crimes but pardoned over a week ago by President Vladimir Putin in a deal that Germany helped broker behind the scenes.

A blast that tore through an electric bus in the southern Russian city of Volgograd during Monday's morning rush hour, killing 14, was probably carried out by suicide bombers from the same organization behind a railway explosion a day earlier, officials said.

Together more than 30 people were killed in the explosions, putting the city of one million on edge and highlighting the terrorist threat Russia is facing as it prepares to host February's Winter Games in Sochi, President Vladimir Putin's pet project. While terrorists may find it hard to get to the tightly guarded Olympic facilities, the bombings have shown they can hit civilian targets elsewhere inRussia with shocking ease.

Fourteen people were killed and 34 injured Sunday when a female suicide bomber blew herself up in a train station in the southern Russian city of Volgograd, investigators said, revising down an earlier toll.

A Volgograd government spokesman had earlier told the state-run RIA Novosti news agency that at least 18 people were killed and more than 40 injured in the attack.

An Argentine Greenpeace activist amnestied by Russia after being thrown in jail for protesting oil drilling in the Arctic demanded an apology from Moscow on Saturday.

Speaking to reporters at Buenos Aires International Airport following her release, Camila Speziale said Russia owed an apology to the 30 activists who were arrested and held in custody for three months.

Three Russian law enforcement officials were killed Friday when a car bomb exploded near a police station in the southern city of Pyatigorsk in the restive North Caucasus region, Russian media reported.

No one claimed immediate responsibility for the attack, but insurgents seeking to establish an Islamic republic along Russia's southern rim frequently target federal soldiers and law enforcement officials.

Russia said on Friday that international experts have agreed a plan for shipping Syria's most dangerous chemical weapons to Italy for their eventual destruction aboard a U.S. ship.

Moscow hosted talks between representatives of the Organization for the Prohibition of Chemical Weapons (OPCW) and disarmament specialists from countries including Syria and the United States aimed at finalizing a plan for removing the war-torn country's chemical arms.