Saudi Arabia's fierce campaign against the Muslim Brotherhood, already on the run in its Egyptian birthplace, has divided a Gulf Arab bloc, causing discomfort in some member states where the Islamist group is embedded in daily politics.
Feuding with Qatar over Islam's place in a turbulent Arab world, Riyadh recalled its ambassador from Doha last week and branded the Brotherhood, a Qatari ally, a terrorist group.
Saudi Arabia has swung firmly behind the Egyptian military, which deposed President Mohamed Morsi last year after mass protests against the Brotherhood leader. Riyadh has since pumped billions of dollars into Egypt's creaking economy.
Punishing Qatar for its pro-Brotherhood stance shows a new Saudi confidence in pushing its agenda, even if it means splits in the six-nation Gulf Cooperation Council (GCC), a U.S.-aligned alliance which usually keeps any internal tensions private.
Bahrain and the United Arab Emirates have followed Riyadh's lead in recalling their envoys from Qatar. But Kuwait, home to an active community of political Islamists integrated into the political and business elite, has voiced disquiet and offered to mediate in the dispute.
Oman, which has opposed plans for a closer Gulf union, appeared to downplay the seriousness of the rift on Wednesday.
"What happened is nothing more than differences and friendly reproach between brothers ... what happened is not a divorce," the Arabic-language Oman Daily cited the Minister of State for Foreign Affairs Youssef bin Alawi bin Abdullah as saying.
The UAE shares Saudi Arabia's view that the Brotherhood, with its taste for populist electoral politics, poses a threat to dynastic rule and to Gulf security, but no other GCC member has yet declared it a terrorist organisation.
Bahrain may be politically close to Riyadh, but Islamist sympathisers are among those who support its own Sunni ruling family against challenges from the island's Shiite majority.
Kuwait must also tread carefully if it is to avoid fuelling chronic strife pitting its hereditary rulers against political opponents who include conservative tribal leaders and Islamists.
"The Saudi announcement is likely to complicate relations with Gulf allies, especially against the backdrop of escalating tensions with Qatar," said research analyst Wafa Alsayed of the Bahrain-based International Institute for Strategic Studies.
"(The terrorist listing) really complicates things because it means anyone could be directly or indirectly subjected to these laws, and someone could be arrested so easily," she said.
Saudi wariness of the Brotherhood dates back at least to the 1990s when some Saudi leaders accused it of inspiring the Sahwa opposition movement agitating for democracy in the kingdom.