European Central Bank head Mario Draghi promised Thursday to do whatever it could to protect the European single currency and sent a strong signal that the central bank has the high borrowing costs that are crippling countries like Spain and Italy in its sights.

Draghi, speaking at a global investment conference hosted by the British government, said that the bank would "do whatever it takes to preserve the euro" and added, "believe me, it will be enough."

Fears about the 17 countries that use the euro have intensified over the past few weeks. Spain and Italy, in particular, are finding it increasingly difficult to raise money on the debt markets due to spiraling borrowing costs. Spain's bond interest rates have hit record highs recently. That has raised fears the country may be the next to see a bailout from the other eurozone countries — after Greece, Ireland, Portugal and Cyprus.

Spain's economy — the eurozone's fourth largest — is much bigger than the other three countries combined and would strain the financing available to the eurozone's bailout funds.

In a question-and-answer session with global business leaders in London for the Olympics, Draghi discussed the high borrowing costs being imposed on some countries' bonds, saying that "they come within our mandate" — as long as those costs stop the ECB's interest rate policies from being implemented across the eurozone.

Stocks across Europe rose on Draghi's comments. Britain's FTSE traded up 1.7 percent, Germany's DAX index rose 2.9 percent and France's CAC 40 rose almost 4 percent. Spain's main IBEX stock index rose 5.5 percent while Italy saw a 5.4 percent rise in its stock prices.

The ECB has already used similar reasoning to make limited purchases of government bonds in the past with the aim of driving down a government's borrowing costs. The bank must walk a careful line, however — because the EU treaty forbids it from using its monetary powers to help government finances directly.

The ECB previously conducted a limited bond purchase program accumulated over 200 billion euros ($245.86 billion) in bonds. But it had little effect on bond yields, and has been left dormant for several months despite pleas from Spain for help.

The eurozone's current bailout fund, the European Financial Stability Facility, had some 440 billion euros in lending power, but most of that is already committed to bailouts for Greece, Ireland and Portugal. A permanent fund, the European Stability Mechanism, would have some 500 billion euros, but is awaiting ratification by eurozone member countries and will not come on line until fall. And 100 billion euros of that is already committed to rescuing Spanish banks from collapsing due to bad real estate loans.

That leaves the ECB once again as the chief crisis fighter, a role it has fulfilled despite the legal restrictions on what it can do. It loaned 1 trillion euros in cheap money to banks in December and February, consisten with its role as lender of last resorts to banks.

Any effort to bail out governments, however, remains deeply controversial.

Many officials and economists, particularly in Germany, are wary of the EU treaty's ban on financing governments. There are also concerned that rescue efforts from the central bank will simply take pressure off politicians to do unpopular things such as cut spending and change labor market rules.