An Egyptian man has pleaded guilty in a New York court to charges of helping to plan the 1998 bombings of US embassies in Kenya and Tanzania that killed 224 people.

Wiping tears from his eyes and frequently shaking his head, Adel Abdul Bary, 54, pleaded guilty to three counts, including threatening to kill, injure or destroy property by means of an explosive and conspiring to murder US citizens abroad.

However, US District Judge Lewis Kaplan did not immediately accept the plea agreement, saying he had concerns that Bary had previously faced hundreds of charges, some of which carried a mandatory life sentence.

Under the plea deal, Bary faces a maximum sentence of 25 years in prison. If he is given credit for more than a decade he spent in custody in the United Kingdom while fighting extradition, his sentence would be significantly shorter, Kaplan said.

"You can well appreciate why I have questions in my mind," Kaplan said.

The judge gave the government and Bary's lawyers one week to submit letters explaining why he should accept the deal. If he were to reject it, Bary would be permitted to withdraw his guilty plea and would proceed to trial.

Assistant US Attorney Sean Buckley told Kaplan that Bary was not alleged to have engaged in any overt acts leading to the murders themselves.

Instead, Bary was largely accused of disseminating claims of responsibility while serving as a conduit between media outlets and senior al-Qaeda figures, including the group's deceased leader Osama bin Laden and his successor, Ayman al-Zawahiri.