Investors were optimistic Tuesday ahead of a much-anticipated Apple event, where the tech giant is expected to announce some new iPhone offerings.

U.S. stock futures were higher ahead of the opening bell.

Shares of Apple (AAPL) inched higher in premarket trading amid expectations the company will release the next generation of iPhones, rumored to be colorful and cheaper to produce. The event is being held at 1 p.m. ET.

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Concerns about a U.S.-led military strike on Syria have been pushed to the back burner, as politicians in Washington continue to debate the pros and cons of a strike. While a military strike had seemed imminent and caused some extreme market volatility in previous sessions, the political process has slowed things down.

Syria "has been focusing people's attention," said Will Hedden, sales trader for IG in London. "We seem to be further away from action, and that's keeping people upbeat."

On Monday, Secretary of State John Kerry raised the possibility that if Syria were to give up its chemical weapons, an attack might be avoided. This opened the door to a diplomatic alternative in Syria, instead of military action.

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The premarket gains follow a strong finish for U.S. stocks Monday. The S&P 500 and Nasdaq each closed about 1% higher. The major U.S. indexes have climbed between 15% and 23% since the start of the year.

European markets were charging higher in morning trading, inspired by bullishness in Asia and North America.

Gains in Asian markets were supported by fresh data from China pointing to a continued economic recovery in the country.