Western diplomats in the Gulf say they are taking very seriously the capture by Bahraini authorities of a boat from Iraq loaded with large quantities of smuggled weapons and explosives.
Bahrain announced on 30 December that it had foiled four separate "terrorist incidents".
The most serious was the interception on the high seas of a speedboat carrying C4 high explosives, mines, grenades and ammunition.
Bahrain's Chief of Public Security, Maj-Gen Tareq Al-Hassan told the BBC: "If this shipment had got through it would have caused mass casualties. It included over 100kg (220lb) of C4 explosive".
Britain has expressed its concern over "the recent terrorist threats faced by Bahrain".
A Western diplomat in the Gulf, who asked not to be named, said this was the biggest counter-terrorism arms haul in two years and it was being taken extremely seriously by Western governments.
'New level of sophistication'
Bahraini officials say that a tipoff from an informant triggered an air-sea operation on 28 December to track the 29ft (9 metres) speedboat 118 nautical miles out in the Gulf as it headed for Bahrain, where it was seized along with two Bahraini crew members.
A second boat was seized heading away from Bahrain's coast carrying 13 wanted suspects, including one Saudi national.
Bahrain has listed the weapons and explosives captured as including:
- 38 blocks of C4
- 31 Claymore mines
- 12 armour-piercing mines
- 6 magnetic mines
- 50 Iranian-made hand grenades
- 295 detonators
- 1 PK machinegun
- Large quantity of ammunition
"This shows a new level of sophistication and professionalism," said Gen Hassan.
"The Explosive Formed Projectiles, especially, have been very professionally packaged. It's the most serious plot to date".
So now the key questions are: who sent it and who was it destined for?
For the past three years Bahrain has been intermittently wracked by clashes between security forces and Shia protesters who complain of systematic discrimination by the Sunni-led government and monarchy.
Most of the protesters are peaceful but there is an increasingly violent fringe that Bahrain accuses of being encouraged and supplied by Iran and its proxy militias.
This seizure of weapons will raise fears that actors outside the country are looking to raise the temperature of Bahrain's insurgency, causing more deaths and injuries.
A Western diplomat said he has seen nothing to indicate any involvement in this arms shipment by al-Wefaq, the main opposition political group.
Commenting on the seizure, a source from al-Wefaq told the BBC:
"We at al-Wefaq are clear that our movement is non-violent and will continue to be so, we will never support armed nor violent conflict.
"The credibility of the authority is always questionable, that's why they face a big challenge to convince anyone, locally or internationally, as they are over-using such allegations trying to link the movement with Iran and Iraq."