Foreign politicians have been congratulating UK Prime Minister David Cameron on the election victory of his governing Conservative Party.

But among the messages of praise are signs of some of the challenges ahead.

The poll was closely watched across the EU, as Cameron has pledged to negotiate a "better deal" for the UK and hold a referendum on membership.

European Commission President Jean-Claude Juncker said he was "looking forward to meeting Cameron soon".

A statement from his office said he would examine any British proposals in "a very polite, friendly and objective way" but warned that key principles including the freedom of movement were "non-negotiable".

In other international reaction:

Matthew Barzun, the US ambassador to the UK, tweeted: "We will work as closely as ever with new government"

Japan's Prime Minister Shinzo Abe called the win "splendid"

Indian Prime Minister Narendra Modi tweeted congratulations, and referenced a Hindi campaign slogan adopted by Cameron to woo voters from an Indian background

Israeli Prime Minster Benjamin Netanyahu tweeted: "I look forward to working with you on shared goals of peace & prosperity"

Russia said it was watching results closely, but said it regretted that "relations are somewhat frozen at present"

The congratulations have been echoed in Europe. Spanish Prime Minister Mariano Rajoy of the Conservative People's Party, tweeting in Spanish, called the victory a "deserved recognition of his decisive reforms".

"A stellar performance. Goodbye polls, hello five more years of government,"commented Alexander Stubb, the Finnish prime minister.

Former centre-right French President Nicolas Sarkozy, who is mounting a political comeback, called it an "impressive victory".