Former US President George HW Bush said the world had lost "a true friend of freedom".
In comments Friday, current German Chancellor Angela Merkel - Kohl's one-time protege and later political adversary - gestured toward these challenges, saying Kohl's memory offers a crucial guide.
His legacy, however, was somewhat tarnished by financial scandal within his CDU party that came to light in 1999.
Kohl died in his house in Ludwigshafen, in the southwestern state of Rhineland-Palatinate, said Bild daily, whose management had close ties with the conservative politician. "Throughout our endeavors, Helmut was a rock - both steady and strong", Bush said Friday. He also chaired the Christian Democratic Union of Germany for many years.
Chancellor Angela Merkel told reporters while on a visit to Rome that Kohl had displayed "exemplary statesmanship in the service of people and peace".
A towering figure in modern German and European history, he was instrumental in the reunification of his country after the collapse of Communism in eastern Europe.
In the background are German government spokesman Hans Klein, West German Finance Minister Theo Waigel, sixth from left, Raissa Gorbacheva, Soviet Finance Minister Pavlov and Soviet Foreign Minister Eduard Shevardnadze, third from right.
"All that has happened in the past 27 years from then until today would have been unimaginable without Helmut Kohl", said Merkel, dressed in black.More news: Death toll from London inferno rises to 58
In 2011, President Bill Clinton called Kohl "the most important European statesman since World War II", adding that "the 21st century in Europe really began on his watch".
European Commission President Jean-Claude Juncker said Kohl "was a great European and a great friend". Working closely with allies like President Ronald Regan, Kohl went from leading West Germany, to having the Berlin Wall torn down, and becoming Chancellor of a once again, reunited Germany.
Former Soviet leader Mikhail Gorbachev underlined Kohl s role in ending the Cold War, noting that "he was without doubt an exceptional personality who left his mark on German, European and global history".
Once viewed as a provincial bumbler, Kohl combined an understanding of the worries of ordinary Germans with a hunger for power, getting elected four times.
President Trump said Kohl was a friend and ally of the United States.
Kohl served as the sixth Chancellor in 1982-1998, being the longest serving German chancellor since Otto von Bismarck.
Putin was a KGB officer in communist East Germany when Kohl spearheaded the reunification of East Germany and West Germany.
The former chancellor was married for 41 years to Hannelore Renner, an interpreter of English and French who stood firmly but discreetly by his side throughout his political life.
Mikhail Gorbachev, who was in charge of the Soviet Union when the Berlin Wall fell in 1989, echoed the sentiments.