Fragile hopes for an end to South Sudan's civil war are being tested by the rebel leader's failure to return to the capital to form a unity government.

International pressure is growing after Riek Machar, a former rebel leader turned deputy president who was fired, became a rebel leader again and has now fought his way back to the vice presidency, failed to appear in Juba as expected on Monday or Tuesday.

"The agreement is at risk," said Festus Mogae, head of the Joint Monitoring and Evaluation Commission (JMEC) established to supervise an August 2015 peace deal that calls for the forming of a so-called unity government led by President Salva Kiir, with Machar as first vice president.

In a statement issued on Tuesday evening Mogae urged both sides to "urgently demonstrate flexibility."

The UN and US also weighed in.

The UN Security Council expressed "serious concern", with its members calling for "all parties to quickly form the transitional government and fully implement the peace agreement," said Chinese Deputy Ambassador Wu Haitao, whose country holds the council presidency.

"The United States is extremely disappointed that Riek Machar has not fulfilled his commitments under the peace agreement and returned to Juba as he stated publicly he would," said US Deputy Ambassador David Pressman late Tuesday.

The latest sticking point appears to be Machar's desire to return to Juba with a large armed entourage which the government said contravenes the peace deal.

Under that agreement, Machar is to join Kiir in a new 30-month transitional government leading to elections. The deal specifies that only 1,370 armed rebels and 3,420 government troops will be permitted within 25 kilometres (15 miles) of the capital.

Tens of thousands of people have died in the fighting since December 2013 and more than two million have been driven from their homes in a conflict characterised by extreme brutality and human rights violations that has split the country along old ethnic fissures.

Fighting has continued despite the signing of the peace agreement which remains largely unimplemented. Machar's homecoming is seen by many as an important step towards shoring up the fumbling deal.

South Sudan's information minister Michael Makuei said Tuesday that the government had blocked Machar's flight because he wanted to bring "machine guns and laser-guided missiles" as well as additional troops.

"This is a stalemate," Makuei said.

On Wednesday Machar in turn blamed the government for the delays saying his 260 additional security personnel are only carrying "light weapons, personal weapons and light machine guns".

"I want to go to Juba," Machar told Al Jazeera. "They are obstructing us."

Mogae, a former Botswanan president who heads the internationally-backed JMEC body said he hoped Machar's return, "could be rescheduled within days, without further conditions."