The United Nations Security Council expressed serious concern on Tuesday over the failure of South Sudanese rebel leader Riek Machar to return to Juba to take up his old post as deputy to President Salva Kiir as part of a peace deal.

U.N. peacekeeping chief Herve Ladsous briefed the 15-member council on South Sudan at the request of the United States. Machar had been due to return to Juba on Monday, but a spokesman for his group said logistical reasons had delayed his travel.

Thousands have been killed and more than 2 million people in a country of 11 million have been driven from their homes by more than two years of fighting that erupted at the end of 2013, barely two years after South Sudan's independence.

Kiir's decision to sack Machar as his deputy in 2013 precipitated the crisis that erupted into conflict in December that year. Fighting has often run along ethnic lines, pitting Kiir's dominant Dinka ethnic group against Machar's Nuer.

Machar and Kiir signed a peace deal in August that called for a transitional government and other security arrangements to end the fighting. But clashes have flared outside the capital and Machar has repeatedly delayed his return.

"The members of the Security Council ... reiterated that they are ready to address any obstruction of implementation of the agreement," China's deputy U.N. ambassador, Wu Haitao, told reporters. China is president of the council for April.

Russian Deputy U.N. Ambassador Petr Iliichev said Ladsous told the council there was a huge lack of confidence between the parties but that the U.N. peacekeeping chief was hopeful that Machar might return to Juba on Wednesday.

U.N. Secretary General Ban Ki-moon spoke to both Kiir and Machar by phone on Sunday and urged them to form a transitional government quickly and roll out other parts of the fragile peace deal.

The United States voiced dismay on Tuesday at Machar's failure to return to Juba.

"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," U.S. Deputy U.N. Ambassador David Pressman said.

"So many international partners - including the U.N. and multiple member states - have undertaken significant political and logistical efforts to facilitate his return," he said.