Viktor Yanukovych, the Ukrainian president, has promised a swift investigation into Friday's bomb blasts in the city of Dnipropetrovsk that injured at least 26 people just weeks ahead of the Euro 2012 football tournament which the country co-hosts together with Poland.
"We understand well that we must find the criminals as soon as possible and they must be punished," Yanukovich said after meeting top law enforcement officials in Dnipropetrovsk where he arrived on Saturday.
"We must understand what motivations and goals this crime had."
Yanukovich said city authorities would pay 2 million hryvnias ($250,000) for any information that would help find the bombers.
Four bombs planted in trash bins in various downtown locations exploded at short intervals in the city of 1.3 million on Friday afternoon, in what prosecutors said was an "act of terrorism".
The president and other officials declined to say what direction the investigation was taking.
The attack, a rare event in the peaceful ex-Soviet republic, has unsettled authorities who are preparing to host the football championship, which starts on June 8.
Ukraine's healthcare ministry said in a statement on Saturday that 22 people were still in hospital, three of them in grave condition.
'Smooth and festive tournament'
The Ukrainian security service has appealed for "citizens who have any information on the circumstances of the explosions and the people involved in the crime" to call specially set-up hotlines.
Authorities denied local media reports that a group had claimed the blasts.
"No demands or threats from identified or non-identified people have been sent to the police," Interfax news agency quoted Vladimir Rakitsky, Ukraine's deputy security service chief, as telling parliament.
Europe's football governing body UEFA has said it is confident Ukraine will be able to ensure security despite the attacks.
"I think the people who committed this brutal crime... are also accomplices to an attack on the image of our country ahead of the Euro-2012," Hryhory Surkis, Ukrainian football federation head, said in a statement.
UEFA, European soccer's governing body, said it remained confident in the Ukrainian authorities' ability to stage a "smooth and festive tournament" despite the bombings.
Ukraine's neighbour, Russia, offered to help the authorities investigate the bombings.
Catherine Ashton, EU's foreign policy chief, said in a statement that the European Union was in close contact with Ukraine and was ready to provide assistance.
Dnipropetrovsk, 400km southeast of the capital Kiev, is one of the former Soviet republic's biggest industrial hubs and was a key centre of the nuclear, arms and space industries in Soviet times.
The city provided a springboard for former President Leonid Kuchma, who was in office from 1994-2005, to rise to power.
Dnipropetrovsk is also the home town of Yanukovych's fierce opponent, the 2004 Orange Revolution leader Yulia Tymoshenko, who is serving a jail sentence for abuse of power that has strained relations between Ukraine and the European Union.
Tymoshenko, 51, has been on hunger strike since last Friday and has said she suffered a beating in the prison where she is serving her disputed seven-year sentence.