ANKARA, Turkey (AP) - Four more people have been detained in connection with two car bomb attacks that killed dozens in a Turkish town near the Syrian border, bringing the number of suspects in custody to 13, Turkey's prime minister said Tuesday.
Syria again rejected Turkey's contention it was involved, condemning the attacks and offering to conduct a joint investigation of an attack it has blamed on Turkey.
Police were still searching for six other wanted suspects, Interior Minister Muammer Guler said. Turkish authorities have blamed the attack on a Marxist group with alleged links to the Syrian intelligence agency, but have not named the group.
Turkish Prime Minister Recep Tayyip Erdogan said the death toll on Tuesday stood at 51 people, including five Syrians.
Saturday's powerful bombings were the deadliest in Turkey in years, shattering the border town of Reyhanli, a main hub for Syrian refugees and rebels fighting against Syrian President Bashar Assad's regime.
In Washington, the White House condemned the bombings, vowing that the attack would only strengthen the U.S. resolve to fight instability and violence in the region.
"We stand with Turkey against such horrific violence," White House spokesman Jay Carney said Tuesday.
Carney said he fully expected the incident would come up Thursday in President Barack Obama's meetings with Erdogan in Washington, along with Syria's civil war, trade issues and broader instability in the Middle East.
A statement carried by Syrian state television following a Cabinet meeting in Damascus put the blame for the bombings on Turkey and its support for the rebels.
"Accusing the Syrian state of these cowardly and terrorist acts is totally baseless," the statement said.
It added: "The Turkish government is responsible for the situation in the border areas through turning them into a haven and passage for the terrorists and allowing gunmen to use the Turkish lands to commit crimes against the Syrians."
Once a close ally of Syria, Turkey has turned into one of the Assad regime's harshest critics. It is a key supporter of the Syrian rebels, offering shelter for many senior and lower-ranking defected Syrian soldiers.
Syria's Information Minister Omran al-Zoubi said later that Erdogan's government was "taking political advantage of Reyhanli's bombings." He said Syria would be willing to take part in a "joint and transparent investigation by special agencies in both countries."
Erdogan, who previously rejected the Syrian denial of involvement as a lie, dismissed the offer of a joint investigation.
"At the moment, there isn't a legitimate administration accepted by the Syrian people," Erdogan said. "There isn't (an administration) we can conduct any joint work with... How can we accept an administration that is not accepted by the people?"
Guler, the Turkish interior minister, said those detained were still being questioned and that authorities were looking for more suspects, including four people who may have been involved in planning the attack and two people suspected of "aiding and abetting" the attackers.
The bombings have sparked speculation that Turkey would be drawn into its neighbor's civil war, but Erdogan reiterated that Turkey would act with restraint.
"Thank God we are powerful, determined and experienced enough to make the culprits pay," Erdogan said. "However, we will act in a cool-headed manner and take steps with the reflexes of a great state."
He said investigators are looking into a possible lack of coordination between Turkey's police and intelligence agency.
Associated Press writers Albert Aji in Damascus, Bassem Mroue in Beirut, and Josh Lederman in Washington contributed.
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