U.S. stock index futures suggest a mixed opening to markets on Thursday ahead of the release of a final estimate of gross domestic product, or GDP, growth for the second quarter, the jobless claims report and pending home sales data, amid continuing uncertainty over Congress’s decisions about the federal budget and a hike in the debt ceiling.

Futures on the Dow Jones Industrial Average were up 0.16 percent, while futures on the Standard & Poor's 500 Index were up 0.12 percent and those on the Nasdaq 100 Index were down 0.41 percent.

A possible government shutdown by Oct. 1 if Congress fails to approve the federal budget and increase the debt ceiling has triggered a sell-off in the markets, with the S&P recording its longest streak of decline in nine months.

On Wednesday, the Senate unanimously passed a stop-gap arrangement to fund the government from Oct. 1 to Dec. 15 and senators will debate the measure, and most likely modify it, before the final draft comes to vote later in the week. The stop-gap measure, first passed by the Republican-controlled House of Representatives, defunds President Barack Obama’s healthcare policy while the Democrat-controlled Senate is expected to modify the bill to allow funding for the project. And, observers believe that the modified measure most likely will be rejected by the House leading to a government shutdown.

“In short, there is a good chance that Congress will fail to agree on the legislation necessary to allow the government to continue with “non-essential” spending when the new fiscal year begins on 1st October. Even if an immediate shutdown is averted, government spending could still run into constraints by mid-October unless the debt ceiling is raised,” Julian Jessop, an economist with Capital Economics, wrote in a research note.

“The uncertainty resulting from another debt-ceiling crisis should, of course, be negative for equity markets,” he added.

Meanwhile, investors are expected focus on the third and final estimate of GDP data for the second quarter, due to be released at 8.30 a.m. EDT on Thursday and analysts polled by Bloomberg expect that GDP growth is likely to be revised to 2.6 percent, fractionally up from the previous estimate of 2.5 percent. 

The GDP data will be closely watched by both the Federal Reserve and financial markets as the former has indicated that the future of the bond-buying program will depend on the strength of economic indicators.

The initial jobless claims report, which measures the number of individuals who filed for unemployment insurance for the first time last week, is scheduled to be released by the Department of Labor at 8:30 a.m. EDT. Economists predict that claims are likely to increase to 330,000 for the week ended Sept. 21, significantly up from 309,000 in the previous week.

Meanwhile, economists expect continuing jobless claims data, which measure the number of unemployed individuals who qualify for benefits under unemployment insurance, to show an increase to 2.840 million from the 2.787 million recorded in the previous week.

The National Association of Realtors' pending home sales index for August will be released at 10 a.m. EDT. The home sales index, which measures the change in the number of homes under contract to be sold but still awaiting the closing transaction, excluding new construction, is likely to decline 1 percent in August after declining 1.3 percent in the prior month.

In Europe, markets traded lower on Thursday with the Stoxx Europe 600 index down 0.28 percent, Germany's DAX-30 was down 0.38 percent and France's CAC-40 was trading down 0.51 percent. London’s FTSE 100 was down 0.2 percent, after data released from the Office for National Statistics showed that UK's economy grew by 0.7 percent on a quarterly basis in the second quarter of 2013, unrevised from the second estimate of GDP, published on Aug. 23, while the annual rate of GDP growth was revised downward by 0.2 percentage points to 1.3 percent from 1.5 percent.

In Asia, markets were mixed, with Japan’s Nikkei ending up 1.22 percent and Australia’s S&P/ASX 200 closing up 0.35 percent. In China, the Shanghai Composite index ended down 1.94 percent while Hong Kong’s Hang Seng Index lost 0.37 percent. South Korea’s KOSPI Composite index added 0.46 percent and India’s BSE Sensex ended up 0.19 percent.