Due to developments over the past few hours, the UN-brokered peace talks between Yemen's warring parties scheduled to start on Monday in Kuwait will be postponed, said Ismail Ould Cheikh Ahmed, UN Special Envoy to Yemen.
"We are working to overcome the latest challenges and ask the delegations to show good faith and participate in the talks in order to reach a peaceful resolution to the crisis in Yemen," Ould Cheikh Ahmed said in statement, without giving details about why the talks were delayed.
Delegations representing the Houthi rebels and the party of former president Ali Abdullah Saleh were expected to arrive in Kuwait on Sunday, but they did not show up.
There have been conflicting reports about the reason why the two delegations had not joined the talks.
"The Houthis decided to wait until they have guarantees that the Saudi-led coalition and the forces loyal to President Abd Rabu Mansour Hadi would abide by the ceasefire," activist Hamzah Al-Kemaly, who is accompanying the Yemeni government delegation, told dpa.
Yemen's government spokesman Rageh Bady told private Saudi broadcaster al-Arabiya al-Hath in a telephone interview that the delay in the talks was due to disagreements between the Houthi and Saleh delegations over calming the situation on the Saudi-Yemeni border.
Fighters loyal to Saleh have been fighting pro-government forces alongside the Houthis.
The mostly Shiite Houthis have not yet issued an official comment, neither has Saleh's General National Congress.
Several breaches of the ceasefire, which had gone into effect on April 10, have been reported on both sides of the conflict.
"Yemen is now at a critical crossroad. One path leads to peace while the other can only worsen the security and humanitarian situation," Ould Cheikh Ahmed said on his official Twitter account.
Meanwhile, hundreds of people in Yemen's volatile city of Taiz protested on Sunday against an ongoing siege, calling on the parties involved in the planned Kuwait peace talks to consider their situation.
"Protesters took to the streets of Taiz to condemn the ongoing blockage imposed on the city by the rebels and forces loyal to Saleh for more than a year as well as the random bombardment of residential districts," activist Maher Al-Absy told dpa.
The central city has for months been a battleground between the forces of Saudi-backed Hadi and Houthi rebels.
Taiz, some 275 kilometres south of Yemen's capital Sana'a, is strategically important because it is located on a road leading to the southern city of Aden, which is Hadi's power base.
Yemen has been in turmoil since September 2014, when the Iran-backed rebels and fighters loyal to Saleh overran Sana'a and began making advances in the country, forcing Hadi to flee to Saudi Arabia.
The Saudis and their fellow Sunni partners launched an air campaign in March 2015 in Yemen against the rebels to reinstate Hadi.
UN efforts have since failed to reach a deal to end war in Yemen, one of the Arab world's poorest countries.