By Kate Davidson 

WASHINGTON--A forward-looking gauge of U.S. home purchases rose for the fourth straight month in April and reached its highest level in nine years--the latest evidence of firming demand in the housing market.

The National Association of Realtors said Thursday its pending home sales index, which is based on contract signings for purchases of previously owned homes, increased 3.4% to a seasonally adjusted level of 112.4 in April from an upwardly revised reading of 108.7 in March.

Economists surveyed by The Wall Street Journal had expected pending home sales would rise 1% in April. Home sales typically close within a couple months after signing.

The index rose 14% in April from a year earlier. That was the eighth consecutive year-over-year increase, NAR said. The index is at its highest level since May 2006.

"Realtors are saying foot traffic remains elevated this spring despite limited--and in some cases severe--inventory shortages in many metro areas," said Lawrence Yun, NAR's chief economist.

News Corp., the owner of The Wall Street Journal, also owns Move Inc., which operates a website and mobile products for the National Association of Realtors.

The gains were broad-based, with all four major regions of the country seeing increases in pending sales.

Sales have stabilized since dipping a year ago, when interest rates on mortgages began to rise from near-record lows. Other recent housing data suggest the spring home-selling season is off to a strong start.

Sales of newly built homes climbed 6.8% in April, to a seasonally adjusted annual rate of 517,000, the Commerce Department said Tuesday.

Completed existing-home sales fell 3.3% in April after surging in March, the Realtors said last week. But the March numbers were revised even higher, and NAR said much of the decline was due to a lack of supply rather than decreased demand from buyers.

Meanwhile, U.S. home-building rose 20.2% in April to the highest level since before the recession officially began, the Commerce Department said last week.

"We are very wary of calling a serious, sustained upturn [in] the housing market, but most of the omens right now are good," Pantheon Macroeconomics Chief Economist Ian Shepherdson said in a note to clients.

Limited inventories have pushed up prices for existing homes. That has helped close the gap in cost between typically more expensive new homes and previously-owned properties. Still, existing home sales account for about 90% of the overall market.

The average interest rate for a 30-year fixed-rate mortgage was 3.67% in April, according to Freddie Mac. Rates are down one-tenth of a percentage point from the previous month and remain low by historical standards.

According to Wednesday's report, pending sales of existing homes in April rose 10.1% in the Northeast, following four straight monthly declines. Pending sales rose 5% in the Midwest, 2.3% in the South and 0.1% in the West.

Write to Kate Davidson at kate.davidson@wsj.com