Tristram Sainsbury, research fellow of G20 studies center at Lowy Institute for International Policy, said both bilateral and regional trade deals could lead to trade diversion rather than trade creation, adding that the world should focus on the multilateral basis.
To reform the international tax system, the leaders endorsed the measures under the Base Erosion and Profit Shifting (BEPS) project, and urged wide and consistent implementation.
The BEPS rules are designed to attack tax avoidance, improve transparency, close loopholes and restrict the use of tax havens.
According to the OECD, once the project is implemented, companies, especially multinationals, will find it harder to concentrate their profits in low-tax countries and tax havens, a shift that promises to raise up to 250 billion U.S. dollars a year in extra tax revenue.
On IMF reforms, the leaders said they are "deeply disappointed" with the continued delay in carrying out the 2010 quota reforms, and urged the United States to ratify these reforms as soon as possible.
Additionally, the leaders also reaffirmed that the they will remain committed to boosting job creation, investment, and promoting cooperation on eradicating poverty, combating corruption, and improving energy efficiency.
CLIMATE CHANGE
Climate change was also a key item on the G20 summit agenda, as preparations pick up for a key international conference on the issue scheduled to start later this month in Paris.
"We recognize that 2015 is a critical year that requires effective, strong and collective action on climate change and its effects," the leaders said. "We affirm our determination to adopt a protocol, another legal instrument or an agreed outcome with legal force under the UNFCCC that is applicable to all parties.
UNFCCC stands for United Nations Framework Convention on Climate Change.
"We affirm that the Paris agreement should be fair, balanced, ambitious, durable and dynamic," the communique said, stressing the commitment to reaching an ambitious agreement in Paris that reflects the principle of common but differentiated responsibilities and respective capabilities."
The leaders said that they will instruct their negotiators to "engage constructively and flexibly" in the coming days to discuss key issues, such as mitigation, adaptation, finance, technology development and transfer and transparency.
FIGHT AGAINST TERROR
The leaders also discussed the fight against terrorism and the refugee crisis.
On the eve of the summit, a wave of terrorist attacks rocked the French capital of Paris, killing 129 people and injuring many more. The Islamic State (IS) militant group has claimed responsibility.
Turkey, the host, has also been subject to terror attacks, the biggest being twin suicide bombings that hit a peace rally in Ankara and killed a total of 102 people on Oct. 10.
In a separate statement, the G20 leaders condemned the "heinous" attacks in Paris and in Ankara, and reaffirmed that "terrorism cannot and should not be associated with any religion, nationality, civilization or ethnic group."
"We remain united in combating terrorism," the statement said. "The spread of terrorist organizations and significant rise globally in acts of terrorism directly undermine the maintenance of international peace and security and endangers our ongoing efforts to strengthen the global economy and ensure sustainable growth and development."
