A senior US official has called India's foreign secretary in another attempt by Washington to tamp down anger 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.

India urged the US to drop the charges against Devyani Khobragade of underpaying her housekeeper and visa fraud.

India's tone suggests that Secretary of State John Kerry's expression of regret yesterday over her treatment was not enough.

State Department spokeswoman Marie Harf said: "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." 

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

Ms Khobragade's arrest has provoked anger in India over the treatment of Indians abroad.

External Affairs Minister Salman Khurshid told reporters that New Delhi was not convinced there was a case against Ms 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"We have asked for an explanation for what has happened ...and why, and we have asked for the cases to be dropped and withdrawn immediately," Mr Khurshid told reporters.

Ms Khobragade was released on bail 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 Ms 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," Mr Khurshid said.

"I don't think that justifies treating her like a common criminal."

Washington said yesterday that Mr 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 Mr Kerry had not gone far enough to assuage Indian sensitivities.

"They should clearly apologise 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 Ms Khobragade's treatment, India has withdrawn some privileges given to US diplomats and removed security barriers at the US Embassy in New Delhi.

The prosecutor in the case, Preet Bharara, the US attorney for Manhattan, defended the treatment of the diplomat, asked why there was less sympathy for the nanny and said it was standard practice for any defendant to undergo a full search, "rich or poor, American or not."

India's foreign ministry lambasted Mr Bharara's remarks.

"The action taken against her was not in keeping with the Vienna Convention," spokesman Syed Akbaruddin said.

"There were no courtesies in the treatment that was meted out to the diplomat, under the normal definition of that word in the English language."

The Indian Embassy in Washington, in a written statement, accused the housekeeper, Sangeeta Richard, of blackmail in demanding that she be allowed to change her passport and visa status to work elsewhere.

It also called on US authorities to arrest Ms Richard for stealing cash, a mobile phone and documents from Mr Khobragade.

Mr Bharara, in his statement, said Richard's family had been brought to the United States after legal efforts had begun in India "to silence her, and attempts were made to compel her to return to India."

In a report filed by Ms Khobragade with police in India, the diplomat said the nanny told her in June that she felt "overburdened" by her work and wanted to be free to leave the house between 7pm and 7am.

Ms Khobragade responded that Richard had come to the United States on an official passport and "perhaps government rules do not permit her to stay beyond the limit of consulate premises," according to police and court documents seen by Reuters.

The report alleged that Ms Richard had committed crimes under Indian law by making a "false promise" in order to enter the United States and was duty-bound to surrender her passport the moment she stopped working as a domestic servant for Ms Khobragade.

A court order issued an arrest warrant for Ms Richard, and -according to foreign minister Khurshid - her Indian passport has been revoked.

"Unless she takes asylum she will have to come back, she doesn't have a valid passport," he said.