Crude oil markets suffered a severe blow Monday amid a range of concerns stretching east from Vienna to Beijing with major indices down more than 4 percent.

West Texas Intermediate, the U.S. benchmark, was down more than 4.5 percent in early Monday trading to $54.35 per barrel. Brent crude oil prices lost 5 percent from the previous session to $58.96. Both indices are at their lowest levels since mid April.

Greece missed its June 30 $1.78 billion payment to the International Monetary Fund, defaulting on its debt, after failing to break an impasse with its European creditors. The Greek crisis left crude oil markets bruised as European leaders said that, unlike the latest global financial crisis, the European economy would be able to weather the storm.

Greek voters decided overwhelmingly against an international bailout deal, leaving the status of the eurozone in question. The British Foreign and Commonwealth Office issued a travel advisory Monday warning citizens of the financial dangers in Greece.

"Visitors to Greece should be aware of the possibility that banking services -- including credit card processing and servicing of ATMs -- throughout Greece could potentially become limited at short notice," it warned.

Negotiators working in Vienna, meanwhile, are working on resolving lingering issues standing in the way of a formal nuclear agreement with Iran. More of Iran's oil could make its way to an oversupplied market if sanctions pressure eases, while international ties could turn frosty should negotiators walk away from the table with a deal in hand.

A senior Iranian diplomat said about 70 percent of the deal has been drafted and "only a few items" are left to debate, though the official insisted "all the nuclear activities of Iran will continue."

The White House has said it would walk away if the deal was less than satisfactory.

In China, meanwhile, the government sought to reassure investors wary of a steep drop in share prices, using the weekend to roll out sweeping measures to stabilize market sentiment.

"It is an urgent task to bring the A share market back to a rational track, as volatility of the market is detrimental to the sound and stable development of the capital market," the ruling Communist Party said.

China is a lead world economy and top consumer of crude oil.