WASHINGTON - A senior US official called India's foreign secretary on Thursday in another attempt by Washington to tamp down the furor in India over the arrest and treatment of one of its diplomats in New York.

Under Secretary of State Wendy Sherman spoke with Indian Foreign Secretary Sujatha Singh to stress the importance of the US-Indian ties following the arrest and strip-search of the diplomat while in US custody and to pledge to work through the complex issues of the case.

India urged the United States to drop the charges against Devyani Khobragade of underpaying her housekeeper and visa fraud, suggesting that Secretary of State John Kerry's expression of regret on Wednesday over her treatment was not enough.

"This is an isolated episode. The secretary ... certainly expressed regret that things weren't done differently at times. But what we're focused on ... this is an incredibly important relationship. That has in no way changed," State Department spokeswoman Marie Harf told reporters.

Washington is not putting pressure on US law enforcement to drop the case, she said.

Khobragade's arrest has provoked anger in India over the treatment of Indians abroad and it has fed into a pre-election ferment, with political parties of all colors voicing patriotic outrage.

External Affairs Minister Salman Khurshid told reporters that New Delhi was not convinced there was a case against Khobragade, who he said had been treated like a common criminal.

"We have asked for an explanation for what has happened ... and why, and we have asked for the cases to be dropped and withdrawn immediately," Khurshid told reporters.

Khobragade was released on bail of $250,000 after giving up her passport and pleading not guilty to charges of visa fraud and making false statements about how much she paid the housekeeper, an Indian national. She faces a maximum of 15 years in prison if convicted on both counts.

The US Justice Department confirmed that Khobragade was strip-searched after her arrest. A senior Indian government source has said the interrogation also included a cavity search.

"The worst that can be said about the lady who was involved ... is that she did not comply with the amount that was supposed to be paid under law," Khurshid said. "I don't think that justifies treating her like a common criminal."

Washington said on Wednesday that Kerry had called Indian National Security Adviser Shivshankar Menon to express regret about the case and his concern that it should not hurt the two countries' relationship.

But in New Delhi, government leaders suggested Kerry had not gone far enough to assuage Indian sensitivities.

"They should clearly apologize and accept they have made a mistake. Only then will we be satisfied," Parliamentary Affairs Minister Kamal Nath said. "At a minimum, they must unilaterally withdraw all charges against Devyani and tender an apology."

In response to Khobragade's treatment, India has withdrawn some privileges given to US diplomats and removed security barriers at the US Embassy in New Delhi.