A bomb blast hit an anti-U.S. protest in northern Yemen on Friday, injuring at least 22 people, a rebel group that controls much of the north of the country said.
In a statement, the leader of the Houthi movement - Shi'ite rebels that Yemen's military tried to crush in campaigns in 2004-2009 - said the bombing took place in the province of Saada, on Yemen's northwestern border with Saudi Arabia.
It did not say who it believed carried out the attack.
The conflict with the Houthis is one of several facing Yemen's new president, Abd-Rabbu Mansour Hadi, as he tries to implement a power transfer backed by Riyadh and Washington.
The transition plan is aimed at averting civil war among an army divided between foes and allies of former President Ali Abdullah Saleh.
Mass protests against him last year were coupled with fighting between pro- and anti-Saleh units. Saleh eventually became the fourth veteran Arab leader unseated by "Arab Spring" protests.
The transition plan calls for Hadi and an interim government made up of Saleh's party and opposition blocs to lead Yemen to elections and write a new constitution within two years.
The plan did not include the Houthis, who have held talks with the U.N. envoy attempting to implement the deal aimed at bringing them into a political process.
He and Merkel had sent a joint message of condolence to the families of those killed in the Monday attack during a concert by US pop star Ariana Grande, he added.
Speaking ahead of her departure for a NATO summit in Brussels, May said she would "make clear to President (Donald) Trump that intelligence which is shared between our law enforcement agencies must remain secure".
One woman arrested by armed police on Wednesday was released a few hours later without charge, leaving 10 men still in custody Thursday, including suspected killer Salman Abedi's father and brother in Libya.
Police said they were serving warrants linked to unrelated criminal investigations when they came under gunfire at the Santa Lucia farming estate, some 860 kilometers (530 miles) from Belem, capital of Para state.
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"Trump not welcome" said banners waved by the crowd, which police said numbered around 9,000. Organisers put the size of the demonstration at 12,000.
Prime Minister Bohuslav Sobotka had demanded the sacking of Andrej Babis, the billionaire founder of the sprawling Agrofert conglomerate, accusing him of tax evasion and multiple conflicts of interest.
Attorney general Luisa Ortega said in a public speech that 55 people have been killed so far in the unrest: 52 civilians and three police.
Corruption was widespread under longtime president Zine El Abidine Ben Ali, who was ousted in a 2011 uprising, and has remained endemic since.
Rudd complained to the US Department of Homeland Security and other intelligence branches about leaks to US media of details of the probe into the deadly Monday attack and the bomber's identity that had not yet been made public.
"They are implicated in affairs of corruption and suspected of plotting against state security through incitement and alleged financing of the protest movements in Tataouine and other regions," he said.
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