U.S. stocks ended higher on Monday, extending the rally from Friday as investors hoped that Russia's move to send humanitarian aid to Ukraine would ease tensions between the two countries.

Earlier in the day, NATO chief Anders Fogh Rasmussen warned of a "high probability" that Russia, using the guise of a humanitarian mission, could intervene militarily in Ukraine.

Ukraine has also said that, contrary to Russian reports of de-escalating, Russia has massed 45,000 troops on its border.

Investors did not seem fazed by the reports. The Market Vectors Russia Exchange-Traded Fund (RSX.P), which gives investors exposure to publicly traded companies based in Russia, gained 1.43 percent.

"It seems like U.S. investors who are taking a risk on Russian equities don't deem NATO's statement as a legitimate concern," said Mark Luschini, chief investment strategist at Janney Montgomery Scott in Philadelphia.

Investors with money in Russian equities seem to be "taking Russia's actions at face value, and not interpreting it as a cover for military action," he said.

Gains were broad, with eight of the S&P's 10 primary sector indexes ending higher for the day. Consumer staples shares posted the highest increases as the sector's index .SPCOMS gained 0.8 percent, while energy and utilities shares dragged.

Stocks also climbed after the United States continued air strikes against Islamic State militants in northern Iraq over the weekend and as Israeli and Palestinian negotiators resumed indirect talks mediated by Egypt on Monday.

The Dow Jones industrial average (.DJI) rose 16.05 points, or 0.10 percent, to end at 16,569.98, still below its close at the end of 2013. The S&P 500 (.SPX) gained 5.33 points, or 0.28 percent, to finish at 1,936.92. The Nasdaq Composite (.IXIC) added 30.43 points, or 0.70 percent, to close at 4,401.33.

Shares of Kinder Morgan Inc (KMI.N), the biggest U.S. pipeline company, jumped 9 percent to $39.37. The stock was the S&P 500's biggest percentage gainer in Monday's session after the company said on Sunday that it would put all its publicly traded units under one roof in a $70 billion deal. Kinder Morgan ranked No. 3 among S&P 500 components in contributing to the benchmark index's gain for the day, in terms of index points

The shares of the company's other units - Kinder Morgan Partners (KMP.N), Kinder Morgan Management (KMR.N) and El Paso Pipeline Partners (EPB.N) - also rallied. The JP Morgan Alerian MLP exchange-traded note (AMJ.P), which tracks energy master limited partnerships, rose 3.6 percent.

Investors snapped up shares of companies developing potential treatments for the often fatal Ebola virus disease amid growing calls to expedite research funding and drug approvals.

U.S.-listed shares of Canada-based Tekmira Pharmaceuticals Corp (TKMR.O) soared 15.0 percent to $23.80. The stock of U.S.-based Sarepta Therapeutics Inc (SRPT.O) gained 7.4 percent to $22.66.

Priceline Group Inc (PCLN.O) shares rose 2.2 percent to $1,309.28 after the online travel agency reported quarterly revenue rose 26 percent. [ID:nL4N0QH3UK] More than 2 million shares were traded, almost triple the 50-day moving average volume of 724,115 shares.

The stock of Chiquita Brands International Inc (CQB.N) surged 30.2 percent to $13.10 in its biggest one-day advance since its trading debut in 2002. Juice maker Cutrale Group and Brazilian investment firm Safra Group offered to buy Chiquita for $13 per share in cash.

About 4.7 billion shares traded on all U.S. platforms, according to BATS exchange data, compared with the five-day average of 6.1 billion.