China was the first world power to react Monday to Russian strongman Vladimir Putin's re-election, vowing to push ties to a "higher level", but Germany questioned the fairness of the vote and warned that Moscow would remain "difficult".

Here's a roundup of global reaction:

- 'Cooperative partnership' -

Just a day after Xi Jinping was unanimously re-appointed President, the Chinese leader said Beijing was willing to work with Moscow.

"Currently, the China-Russia comprehensive strategic cooperative partnership is at the best level in history, which sets an example for building a new type of international relations," Xi said in a message to Putin, according to the official Xinhua news agency.

"China is willing to work with Russia to keep promoting China-Russia relations to a higher level."

- 'Challenges' -

German Chancellor Angela Merkel was to congratulate Putin, but the message will also raise "challenges" in their relations, her spokesman said.

Berlin and Moscow have "differences in opinion" on issues ranging from Russian politics to the conflicts in Ukraine and Syria.

"Nevertheless, the continuous contact with Russia's leadership is very important to us."

Foreign Minister Heiko Maas was more critical, saying: "We certainly cannot talk in all respects about a fair political contest as we know it."

It was "unacceptable" that the election also took place in Crimea, which Russia annexed from Ukraine four years ago in breach of international law, Maas said.

"In this respect, we assume that Russia will remain a difficult partner."

Poland also took up the Crimea theme with the foreign ministry saying the vote on the peninsula was "illegal" and urged respect for Ukrainian territorial integrity.

- 'Modernising Russia' -

In a phone call with Putin, French President Emmanuel Macron wished him "success for the political, democratic, economic and social modernisation" of Russia.

But he also urged Putin to shed light on the 'unacceptable' double agent poisoning.

- 'Denuclearise North Korea' -

Japanese Prime Minister Shinzo Abe congratulated Putin and the two leaders agreed to work together for North Korea's denuclearisation, the Japanese foreign ministry said.

"The two leaders confirmed their close cooperation in realising North Korea's denuclearisation" before an expected summit between US President Donald Trump and North Korean leader Kim Jong Un, it said.

Abe and Putin also discussed joint economic activities on disputed islands and "humanitarian measures" for former Japanese islanders.

- 'Exceptional trust' -

"The Russian people's exceptional trust in you is a natural outcome of your outstanding national performance," Syrian President Bashar al-Assad told his close ally in a congratulatory cable.

- 'Develop relations further' -

Iranian President Hassan Rouhani congratulated Putin on his "decisive victory" and pledged to boost ties.

"I am sure that during your new term, relations between our two countries will develop further," he said.

Iran and Russia have strengthened ties in recent years, both giving major military and financial backing to Syrian President Bashar al-Assad.

- 'Strategic ties' -

Egyptian President Abdel Fattah al-Sisi sent his "warmest congratulations," according to a statement.

"(Sisi) lauds the strategic relations that the two countries share and their keenness to further enhance them so as to realise the interests of both peoples."

- 'Brother countries' -

The presidents of leftist regimes in Venezuela and Bolivia effusively congratulated Putin both using the same word, "overwhelming" to describe the victory.

Russia and Venezuela are "brother countries" as both must "face the frequent manoeuvres of imperialism" to "impose doctrines of world supremacy," said President Nicolas Maduro.

Bolivian President Evo Morales said on Twitter that Putin's win "guarantees geopolitical equilibrium and world peace before the onslaught of imperialism".

- 'Health and happiness' -

Saudi Arabia's new strongman, Saudi Crown Prince Mohammed bin Salman and his father King Salman sent congratulations and wished Putin "constant good health and happiness and his people steady progress and prosperity".