NEW YORK (TheStreet) -- Stocks posted robust gains by mid-afternoon Monday as debt discussions between the eurozone and Greece boosted investor sentiment. 

The S&P 500 was up 0.65%, and the Dow Jones Industrial Average gained 0.63%. The Nasdaq added 0.67% to 5,151, down from an earlier all-time intraday high of 5,162.

Fitbit (FIT) extended gains achieved since the company's trading debut last Thursday. Shares added 13.6% to $36.94. The initial public offering of San Francisco-based Fitbit was priced at $20 a share. Click here for more.

Greek Prime Minister Alexis Tsipras submitted new proposed reforms over the weekend, which were a condition of further debt relief. Among the reforms reported, though not confirmed, Greece will eliminate early retirement benefits, cut defense spending and increase corporate tax rates.

Some analysts remain unconvinced the deal is as satisfactory as hoped for, however. 

"We remain apprehensive that even if some type of deal is cobbled together in this the 11th hour, it will be more akin to a can-kicking maneuver that only offers a temporary reprieve for a few months or at best quarters before Greece finds itself once again unable to fulfill the agreed upon terms," CRT Capital analysts said in a note. "But we're certainly not lacking in cynicism about the process at this point."

Eurozone finance ministers had convened for an emergency summit in which to discuss Greece's future in the eurozone at a political level. Talks have escalated as Greece faces a crucial repayment to the International Monetary Fund by the end of the month.

U.S. existing home sales rose 5.1% to 5.35 million in May, the fastest pace since 2009. The share of first-home buyers increased to 32% from 30%. Economists had expected the May rate to increase to 5.25 million, up from a revised pace in April of 5.09 million. 

"This was a very constructive report and it suggests that the U.S. housing market recovery is back on track after the missteps earlier this year," said Millan Mulraine, deputy head of U.S. strategy at TD Securities. "The shift in the sales mix in favor of first-time buyers is a healthy development, and it points to a more solid foundation for the housing recovery going forward."

Cigna (CI) rejected Anthem's (ANTM) increased offer of $184 a share after its board unanimously opposed the terms. The new offer from Anthem had represented a 35.4% premium over Cigna's late May price before reports of a deal surfaced.

The health care sector has been the focus of M&A talk for the past few weeks, including potential matchups between Aetna (AE)T and either Humana (HUM) or UnitedHealth (UNH). The Health Care SPDR ETF (XLV) gained 0.9% on Monday.

JPMorgan (JPM) led the Dow, gaining 1.3%. Other major banks including Wells Fargo (WFC), Goldman Sachs (GS) and Morgan Stanley (MS) were all higher, while the Financial Select Sector SPDR ETF (XLF) gained 1.5%.

Sequential Brands (SQBG) announced that it has agreed to acquire Martha Stewart Living Omnimedia (MSO) for $6.15 a share in a cash-and-stock deal. Martha Stewart shares gave back 12% on Monday after rallying last Thursday once news broke of deal talks.

Tesla (TSLA) moved 1.3% lower on reports production of the Model 3 electronic car had been delayed to 2018, according to Inside EVs and reported by Bloomberg. A Tesla spokesperson reiterated a previous 2017 timeline. Click here for more.

Oil and gas company Williams (WMB) rocketed nearly 25% higher after Energy Transfer Equity (ETE) confirmed it had made an unsolicited bid worth $48 billion. Williams rejected the offer. 

Facebook (FB) climbed nearly 3% on reports is encroaching on Google's (GOOGL) YouTube in terms of market share for online video advertising. Research firm Apere estimated Facebook has an audience size of 1.4 billion monthly active users compared to YouTube's 1.3 billion.