By Todd Buell and Brian Blackstone 

FRANKFURT--The European Central Bank warned in a report Thursday that the Greek debt crisis could spread to other at-risk eurozone countries if Athens fails to reach a financing deal with its international creditors quickly, underscoring the high stakes involved in the country's negotiations with its lenders.

Still, in a news conference ECB Vice President Vitor Constancio played down concerns in financial markets that Greece may ultimately be forced out of the euro if it fails to pay its debts to the International Monetary Fund and other creditors.

A Greek default wouldn't automatically mean that its banks are insolvent, he said, although the ECB would have to include the implications of such an event in its analysis of the financial health of the country's financial institutions that rely heavily on ECB loans for funding.

"Financial market reactions to the developments in Greece have been muted to date, but in the absence of a quick agreement on structural implementation needs, the risk of an upward adjustment of the risk premia demanded on vulnerable euro-area sovereigns could materialize," said the ECB in its Financial Stability Review, which is published every six months.

"Default risk expectations have increased sharply in Greece amid heightened political uncertainty," the report said.

However, "there is no automatic connection between a default of the Greek government and the solvency of the Greek banks," Mr. Constancio said, stressing the word "automatic." He noted that the share of Greek government bonds in total bank capital is quite small. Such an occurrence may influence the ECB's impairment analysis of the exposure of Greek banks to the state, he added.

Mr. Constancio said he was convinced that Greece won't exit the euro.

"It is difficult to build up a narrative where that extreme case can happen," he said. A country cannot legally be expelled from the currency bloc, he noted, adding that a rising share of Greek citizens have said they want to stay in the euro.

The struggling Mediterranean state is currently in protracted negotiations with its eurozone creditors over the future of its bailout deal. While recently, Greek leaders have said that an agreement is near, other eurozone leaders, such as German Finance Minister Wolfgang Schäuble, have shed doubt on such claims.

"We always hear positive news coming out of Greece, which is good. However, we haven't gotten much further in substance in the negotiations between the three institutions and the Greek government," Mr. Schaeuble said on German public broadcaster ARD on Wednesday.

Meanwhile, Greek banks are heavily dependent on ECB loans for funding amid a steady decline in bank deposits there. Mr. Constancio pegged the total amount of support--through regular ECB loans and emergency liquidity via the Greek central bank--at EUR114 billion.

The ECB also said in its financial stability report that "the prospect of an environment of low nominal growth remains the major factor underlying current challenges for financial stability in the euro area."

The ECB again stressed the need for structural changes in Europe, saying that while monetary policy "can support the conditions for economic growth," other policies, such as structural reforms "are needed to underpin sustainable economic growth in the euro area."

"The financial stability situation in Europe has improved," Mr. Constancio said. "One can say that our policies are working."

In March, the ECB started a broad-based asset-buying program, known as quantitative easing, where it will buy EUR60 billion ($65.64 billion) a month of mostly government bonds until the end of September 2016 in an effort to prop up inflation.

Mr. Constancio said he doesn't see evidence of generalized overvaluation in asset markets, but flagged this as a risk.

"The main risk is the possibility of a reversal of asset valuations...that would induce capital losses and disturb the recovery if it would happen," he said.

Andrea Thomas

contributed to this article.

Write to Todd Buell at todd.buell@wsj.com and Brian Blackstone at brian.blackstone@wsj.com