The FBI says that one of the alleged spies accused of being part of a deep cover espionage ring in the US has confessed to working for Russian intelligence and to constructing a fictitious identity. But he has refused to reveal who he really is and said that his loyalty to "the service" is worth more to him than his children.

The revelation by prosecutors came as nine of the alleged spies appeared in court yesterday looking dishevelled, sullen and very unlike the highly trained secret agents the US claims them to be. One of their lawyers said that the evidence against them amounted to no more than proving they had infiltrated cocktail parties and the parent teachers' association.

In a letter to the New York court, prosecutors said that Juan Lazaro, an academic arrested with his wife, Vicky Pelaez, a newspaper columnist, told investigators that he was in the pay of Russian intelligence and that he had used his wife to deliver information. But Lazaro refused to disclose information about his true identity – he passed himself off as Uruguayan but FBI bugs heard him tell his wife about his family fleeing to Siberia during the second world war – or his work for Moscow.

"Although he loved his son, he would not violate his loyalty to 'the service' even for his son," the letter said in describing Lazaro's statement. The letter was submitted in opposition to bail for the alleged spies who made brief appearances at courts in Boston, New York and Virginia.

Prosecutors cited the case of another of the accused, Christopher Metsos, who jumped bail in Cyprus and is the subject of an international manhunt, in emphasising why the nine alleged spies should be kept in detention awaiting trial.

"The charges in this case have revealed that vast swaths of the conspirators' lives in the United States have been, simply, fraudulent," the prosecution said. "There is little need here for speculation as to what will happen if the defendants are permitted to walk out of the court. As Metsos did, they will flee."

The 11 accused are charged under a rarely used law of working for a foreign government without registering with the US authorities. If convicted, they face up to five years in prison. Nine of them also face money laundering charges, which carry longer sentences.

First to appear at a court in Boston were a couple charged under their assumed names, Donald Heathfield and his wife, Tracey Lee Ann Foley.

Like the other defendants, they were addressed by the names they used as cover – in Donald Heathfield's case stolen from a Canadian who died in 1963 at six months old – because the FBI has still not been able to establish their true identities.

As Foley entered the court in handcuffs and leg shackles, she smiled at her sons, Timothy, 21, and Alex, 16. The boys waved back. Their father acknowledged them only at the end of the hearing when he gave them a firm nod and they nodded back.

Heathfield's lawyer, Peter Krupp, said afterwards that there is no substantial evidence against his client.

"It's extremely thin. It essentially suggests that they successfully infiltrated neighbourhoods, cocktail parties and the PTA. My client looks forward to facing the charges," he said.

Their bail hearing was postponed for a fortnight because the defence said it needed more time to prepare. But later in the day the couple were allowed to meet for the first time since their arrest to discuss who should have custody of their younger son while they are imprisoned.

At a court in Alexandria, Virginia, another couple, Michael Zottoli and Patricia Mills, appeared in dark green jail garb with the word "prisoner" stamped on the back. She stared straight ahead and said nothing to the man she had described as her husband and with whom she has two young children who are now in the care of social services. Both appeared exhausted and subdued.

The judge agreed to postpone the bail hearing until today after a defence lawyer said he had just been told "new information from the government" which he did not disclose although sources later said it was not related to the prosecution's letter.

Shortly afterwards another alleged spy, Mikhail Semenko, made a brief appearance. Semenko, who lived in Arlington and drove a top end Mercedes, asked the court to appoint him a lawyer.

The New York court released Pelaez on $250,000 bail although she will be subject to house arrest and electronic monitoring. Lazaro's bail hearing was postponed at the request of his lawyers.

But the court denied bail for two alleged spies based in New Jersey, who called themselves Richard and Cynthia Murphy – and who attempted to establish ties to financial and political figures – because the judge said the evidence showed that their identities were fake and their real background a mystery. However it is known that the Murphys have two young daughters who have been taken in to care.

As questions continued to dog a case that has been met with a mix of mirth, scepticism and outrage in the US, prosecutors in New York said that they had far more evidence against the Russian spy ring than had so far been published.

In their letter to the court, prosecutors said that they had so far revealed only a fraction of the incriminating evidence including scores more intercepted messages between the alleged spies their Russian handlers. The revelation of further evidence may have been prompted in part by widespread doubts in the US that the supposed spy ring should be taken as a serious threat to national security. Increasingly, the alleged Russian agents are seen as incompetent and out of date amid widespread questioning of why they used such elaborate undercover methods apparently in search of the kind of information that is often in the public domain.

Another alleged spy, Anna Chapman, was denied bail on Monday. MI5 is continuing to investigate Chapman's activities in Britain, according to government sources. But officials said there was no evidence she posed a threat to the UK's national security.

• This article was amended on 2 July 2010. Originally, a heading said that a suspect was reported to have put loyalty "to the KGB" before his family. This has been corrected.