Bahraini police have arrested four suspects in connection with bombings that killed two Asian expatriates in the capital Manama, the official BNA news agency reported on Tuesday.
The news agency did not elaborate on when or how the arrests were made.
"An investigation is under way to uncover the circumstances surrounding these terrorist crimes and identify the rest of the criminals and arrest them," BNA quoted public security chief Major-General Tariq al-Hassan as saying.
The report came hours after King Hamad ordered "the swift arrest of the terrorists who carried out the recent terrorist acts in Bahrain."
The king appealed to the public for help to "bring them to justice so they receive their punishment over this appalling act."
Prime Minister Sheikh Khalifa bin Salman al-Khalifa said: "The terrorists and vandals plotting against Bahrain will receive their punishment and the inciters will be held responsible for what happened.
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"The government will not allow a stray group to destroy the nation, threaten and terrorise safe citizens," he added.
Five bomb blasts in the capital's Gudaibiya and Adliya districts killed two Asian expatriates and wounded a third on Monday, police said.
The bombings came amid persistent tensions between the kingdom's Shiite majority and its Sunni rulers since the bloody crushing of month-long pro-democracy protests in March last year.
The main Shiite opposition group Al-Wefaq condemned the attacks but cautioned that "due to the absence of independent human rights and media groups, it is difficult to clearly determine the truth behind the incidents."
The United Nations on Tuesday condemned the bombings. "These violent acts cannot be justified by any cause. We call on all concerned to exercise maximum restraint and to refrain from any provocations," said UN spokesman Martin Nesirky.
"We urge all Bahrainis to come together in a spirit of national unity and to resolve differences peacefully through dialogue and reconciliation."