UN and Lebanese forces work together to monitor the south
An arms cache which exploded last week in Hezbollah-controlled territory in south Lebanon was "very active", a senior UN official has told the BBC.
The official said the existence of the cache was a "serious violation" of the UN resolution that ended the 2006 Israel-Hezbollah war.
Hezbollah says the blast was caused by old shells collected after the war.
A US official also accused Hezbollah of impeding a UN probe into the cache, after troops were pelted with stones.
Speaking anonymously, the official said the incident was the most serious since the end of the war and that he was seriously concerned about the possible consequences.
The UN official told the BBC that the weapons included Israeli-manufactured arms, but said it was "normal for the armies to keep the weapons of the other side".
Last week, a day after the explosion, 14 UN peacekeepers were injured when a crowd of demonstrators threw stones and tried to prevent them from reaching the site.
Speaking after a behind-closed-doors UN briefing, Alejandro Wolff, the US deputy ambassador to the UN, said the stone-throwing seemed to be "quite organised" and "not spontaneous".
And he said there were "indications" of "providing cover for efforts to remove evidence".
Israel has filed an official complaint to the UN, accusing Lebanon of violating UN Security Council Resolution 1701.
The resolution was passed in 2006 and ended the 34-day war between Israel and Hezbollah.
It banned all unauthorised weapons and military activity between the Litani River and the UN-monitored border between Israel and Lebanon.
The explosion took place in an abandoned building in Khirbet Silim, about 9 miles (15 km) from the Israeli border.