CAIRO, Egypt -- A top U.S. diplomat held talks with a jailed senior leader of the Muslim Brotherhood on Monday as part of mediation efforts to end the standoff between Egypt's military-backed government and protesters supporting ousted President Mohammed Morsi, Egyptian officials said.

The talks between U.S. Deputy Secretary of State William Burns and Khairat el-Shater, the powerful deputy head of the Brotherhood, took place in the prison were the Islamist figure is being held, the officials said on condition of anonymity because they were not authorized to brief the media.

Burns was accompanied by the foreign ministers of Qatar and the United Arab Emirates as well as an EU envoy. U.S. Embassy spokeswoman Patricia Kabra declined comment, but a spokesman for interim Egyptian President Adly Mansour confirmed the meeting. He said the four were also due to meet later Monday with another detained Brotherhood leader, Saad el-Katatni.

El-Shater was among a host of prominent Islamists arrested by authorities after the army ousted Morsi, a longtime Brotherhood member, on July 3. He has been charged with complicity in the killing of anti-Morsi protesters during the four days of protests that led up to the military coup.

The government officials did not say why Burns and the other diplomats visited el-Shater, who was widely believed along with the Brotherhood's spiritual leader Mohammed Badie to be the source of real power during Morsi's one year in power.

But Burns and the three other diplomats are in Egypt as part of international efforts to end a standoff between Morsi's supporters and the military-backed interim government. Also Monday, senior U.S. senators John McCain and Lindsey Graham arrived in Cairo at President Barack Obama's request to press for a quick return to civilian rule.

More than a month after Morsi's ouster, thousands of the Islamist leader's supporters remain camped out in two key squares in Cairo demanding his reinstatement. Egypt's military-backed interim leadership has issued a string of warnings for them to disperse or security forces will move in, setting the stage for a potential violent showdown.

Already, some 250 people have been killed in violence since Morsi's ouster, including at least 130 in two major clashes between security forces and supporters of the deposed president on July 8 and again on July 26 and early the next day.

In Brussels, an EU official said the main challenge at the moment was to build trust between the two sides so they can eventually sit down and talk.

"We don't want to raise expectations at this moment. We are still working at the level of building confidence between the various sides," the official said on condition of anonymity because the talks were ongoing.

The official said such confidence-building measures could include releasing people, dropping charges against Brotherhood figures and breaking up the sit-ins.

Morsi, Egypt's first freely elected president, has been held at an undisclosed location since his removal from office. Last week, he was visited by the EU's foreign policy chief Catherine Ashton and a group of African statesmen. Ashton said he was well and had access to TV and newspapers.

Morsi faces accusations of conspiring with the militant Palestinian Hamas group to escape prison in 2011.

El-Shater, Badie and four others, meanwhile, are to go on trial on Aug. 25 on charges related to the killing of eight protesters outside the Brotherhood's Cairo headquarters during the mass protests leading up to the coup. Badie remains in hiding.

In a brief statement, the Brotherhood said Morsi remained the legitimately elected president who should be spoken to and not anyone else. It did not, however, condemn the Burns visit. Gehad el-Haddad, a Brotherhood spokesman, reinforced that position.

"Our position of full reversal of military coup is unchanged," he wrote on his Twitter account.

The visit to el-Shater in prison came after Egypt's highest security body, which is led by the interim president and includes top Cabinet ministers, announced that the timeframe for any negotiated resolution to the current standoff should be "defined and limited."

It also called on the pro-Morsi protesters to abandon their sit-ins and join the political road map announced the day of the coup.

With the Islamist-backed constitution adopted last year suspended and the legislature dominated by Morsi's supporters dissolved, the road map provides for a new or an amended constitution to be put to a national referendum later this year and presidential and parliamentary elections early in 2014.

Mansour's spokesman, Ahmed el-Muslemani, said not a "single step" of the road map would be changed and ruled out a referendum on the transition plan. His comments appeared designed to debunk speculation that the flurry of diplomatic visits is likely to persuade Egypt's interim government to offer substantive concessions to Morsi's supporters in return for an end to the sit-in protests.

Burns had extended his visit to Cairo by two days so he could have further talks with Egyptian leaders on Sunday and Monday. He met Defence Minister Gen. Abdel-Fatah el-Sissi, who led the July 3 coup, and the prime minister on Sunday. Kabra, the U.S. Embassy spokeswoman, would not say whether Burns intended to further extend his stay.

A rare piece of good news amid the turmoil came from the Central Bank Sunday when it announced that foreign reserves rose to their highest level in nearly two years, reaching $18.8 billion at the end of July or a nearly $4 billion jump from $14.9 billion at the end of June.

The bank did not provide details on the influx of cash, but the roughly 26 per cent rise from the previous month comes after oil-rich Gulf Arab countries pledged billions of dollars in aid to Egypt's interim government in the wake of the July 3 coup.

The new reserve figures are their highest since November 2011. While July's reserve figures represent a significant boost, they are still roughly half of what they were prior to the 2011 uprising that ousted longtime President Hosni Mubarak from power.