Most frontlines in the Yemeni civil war were reported to be calm after a ceasefire between mainly Shiite rebels and a Saudi-led coalition backing their local rivals came into effect at midnight (2100 GMT).
Local residents reported shelling by the Houthi rebels on residential areas in the flashpoint central city of Taiz early Monday, while coalition air raids were reported there and in the neighbouring southern province of Lahj.
Both the mainly Shiite Houthis and a Saudi-led military coalition backing their local rivals reaffirmed their commitment to implementing the UN-brokered ceasefire shortly before it came into force.
The truce comes ahead of UN-brokered peace talks due to start April 18 in Kuwait, where the rebels, who control the capital Sana'a, will discuss peace terms with the government of internationally recognized President Abd Rabu Mansour Hadi.
Aid agencies have said the ceasefire is a critical moment for the already poverty-stricken country, which is now seeing thousands of civilians flee fighting every day.
In Taiz, residents and local journalists said the rebels had shelled several residential areas and a military base after midnight.
Hadi loyalists in the city said they had observed 12 rebel breaches of the ceasefire. Houthi spokesmen could not immediately be reached for comment on the claims.
In the neighbouring province of Lahj, residents said coalition warplanes had struck Houthi artillery positions after the rebels fired on loyalist positions shortly after midnight.
The rebels' Al Masirah television said the coalition had also bombed an area in Taiz. Residents said an air raid hit an anti-aircraft battery.
The 24 hours before the ceasefire came into effect had seen coalition jets bombing locations in Sana'a as well as heavy fighting on the ground in the Taiz region.
A coalition of international aid groups earlier said that the ceasefire would be "a moment of truth" for the nation.
The conflict has led to warnings of disastrous humanitarian consequences, including food shortages, in what was already one of the poorest countries in the Arab world.
The Saudi-led air campaign, ongoing since March last year, has been condemned by human rights groups for its impact on civilians, with the UN also saying that it has caused most of the conflict's civilian casualties.
"A real ceasefire could be the first step towards ending this staggering yet forgotten crisis," said Jan Egeland, head of the Norwegian Refugee Council, one of the groups issuing the statement.
"This last year of escalated violence has meant that each day, around 6,610 people were forced to flee their homes and around 25 civilians are killed or injured," the watchdogs said in Sunday's joint statement.
Previous ceasefires in Yemen have not held, but a partial truce covering the border with Saudi Arabia, announced last month, has been largely effective.
However, a surprise reshuffle by Hadi last Sunday has raised fears that the government side - which has very limited control over the forces fighting the Houthis on the ground - is not in a mood for concessions.
Hadi replaced his conciliatory vice president Khaled Bahah with hardline former military commander Ali Muhsin al-Ahmar, who led several campaigns against the Houthis since their initial uprising in 2004.
"At face value, this would seem to be a signal that the government is moving away from a political settlement, and preparing to escalate the conflict," analyst Adam Baron of the European Council on Foreign Relation told dpa.
The Saudis, fearing that the rebels would give their regional rival Iran a foothold in the Arabian Peninsula, launched their air campaign last March when the Houthis moved on Hadi's heartland in southern Yemen.
A loose alliance of Hadi loyalists, Sunni Islamists, jihadists and southern Yemeni separatists, backed by Gulf troops and air power, has since pushed the rebels back out of the south.
But they remain entrenched in the north, while inconclusive fighting has continued in central Yemen.
The Yemeni branch of al-Qaeda has been one of the war's main beneficiaries, fighting alongside anti-Houthi forces in some areas while also seizing territory for itself behind their lines.
A fledgling branch of the Islamic State extremist group has also made a grisly debut, mainly in the form of deadly attacks on mosques in the Houthi-held capital.