15 February 2011 Last updated at 23:36 ET
US Senator John Kerry has said a US man arrested for killing two Pakistani men will be subjected to a criminal inquiry at home if Islamabad released him.
Senator Kerry also expressed regret over the killings which have soured relations between the two countries.
The US official arrived in Pakistan on Tuesday in a bid to resolve the row sparked by the arrest of Raymond Davis for last month's shooting in Lahore.
Mr Davis was taken into custody after confessing to the shootings.
Mr Davis has said he shot the two men in self-defence as they were trying to rob him.
The US government has announced that it would prove before a Pakistani court later this week that Mr Davis has diplomatic immunity.
"It is customary in an incident like this for our government to conduct a criminal investigation. That is our law. And I can give you the full assurance of our government today that that will take place," Associated Press quoted Senator Kerry as telling reporters in Lahore.
"So there is no such thing as a suggestion that something is out of law or that America thinks somehow we're not subject to the law," he said.
The case is threatening to derail US ties with Pakistan, a crucial ally in the fight against militants, and Senator Kerry's visit is an attempt to smooth over relations.
"I want to come here today to express our deepest regret for this tragic event and to express the sorrow of the American people for the loss of life that has taken place," he said.
Mr Kerry's arrival in Pakistan coincided with President Barack Obama saying that Mr Davis should have diplomatic immunity.
The president said it would be untenable if diplomats were prosecuted.
Mr Davis is charged on two counts - murder and possession of illegal weapons.
The court has ordered the Pakistani government to clarify US embassy claims that he has immunity.
Mr Davis has reportedly been sent to the high-security Kot Lakhpat jail in Lahore. His next hearing is scheduled for 25 February.
It is not clear what his role in Lahore was - American officials in the capital Islamabad have said only that he was an US embassy employee who was part of the "administrative and technical staff".
The Associated Press news agency says Pentagon records show that Mr Davis is a former Special Forces soldier who left the army in 2003 after 10 years of service.
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