The UAE said 45 of its soldiers engaged in a Saudi-led campaign against Shia rebels in Yemen were killed in an accidental explosion that the insurgents claimed was caused by rocket fire.
In what was the deadliest day for the coalition since it was formed in March, Bahrain said five of its soldiers guarding the Saudi border with Yemen were also killed.
The Yemeni government said an "accidential explosion" at an arms depot at a military base in the eastern province of Marib killed the Emiratis, but the rebels said their fighters fired a rocket that caused the blast.
Coalition ally Bahrain said five of its soldiers were killed in southern Saudi Arabia where they had been posted to help defend the border with war-wracked Yemen. It did not give a precise location.
However, Yemen's exiled presidency said the Bahrainis died in the same blast that killed the Emiratis.
The Arab coalition has battled Iran-backed Houthi rebels to restore the rule of President Abedrabbo Mansour Hadi, exiled in Riyadh.
Around 60 people, mainly military personnel, have died in cross-border rebel attacks in the south of the Saudi kingdom since the coalition began air strikes on the Houthis and their allies.
The campaign began as the Houthis advanced on the southern port of Aden, after they took control of the capital Sanaa last September.
The UAE armed forces, in a statement carried by state news agency WAM, did not disclose the circumstances of what was its highest casualty toll of the six-month-old air war.
The Emirati army had previously announced at least eight deaths in Yemen among its ranks.
Before the Emirati toll rose, the pro-Hadi army command said a total of 33 Yemeni soldiers and coalition forces were killed and dozens wounded in the blast at Safer, 250 kilometres (150 miles) from Sanaa.
A thick plume of black smoke was still billowing from the base several hours later.
Friday's coalition losses came as Saudi King Salman was in Washington for talks with US President Barack Obama at which Yemen figured high on the agenda.
Obama said the two sides "share concerns" about the need to restore a functioning government in Yemen and relieve an urgent humanitarian crisis.
The United States has supported the coalition effort, but repeatedly warned about the effect the fighting has had on civilians.
More than 4,500 people have been killed in the conflict, including hundreds of children, according to the United Nations, which has warned that the impoverished country is on the brink of famine.
According to military sources, the coalition sent reinforcements to the Safer base this week, including tanks, armoured vehicles, troop carriers, rocket launchers and Apache helicopters.
The extra hardware and troop reinforcements aim to boost "the counter-offensive launched by loyalist forces and the coalition to advance on Sanaa", one military official in Yemen said.
The Houthis, meanwhile, said their forces had killed "dozens of officers and soldiers of the mercenaries in the Saudi aggression" when they fired a Tochka ballistic missile at the camp.
The strike was "revenge for the crimes and the war of extermination being carried out by the Saudi aggressor and its mercenaries", they said.
The government denied the Houthi account, saying the explosion near an Emirati encampment in Safer was caused by "badly stored munitions".
However, one Yemeni military source told AFP an initial investigation found that the blast was caused by a surface-to-surface missile fired by the rebels.
The Houthis, who advanced from Sanaa to overrun large chunks of the country, were driven out of Aden in July.
They have since been on the defensive in southern Yemen, losing control of several provinces.
The Arab coalition has also sent in troops, with Saudi media reporting that roughly 1,500 soldiers, most from the UAE, had entered Aden.
The UAE only confirmed last month that its troops were on the ground there.
The blast in Safer came as loyalist forces kept up their drive to claw back territory lost to the Houthis over the past year.
Coalition warplanes later on Friday carried out air strikes on the rebel-held defence ministry complex in Sanaa and also targeted arms depots in the north of the capital, witnesses said.