The integration of Generative Artificial Intelligence (GAI) into English language teaching and learning, particularly in second language (L2) writing, has been widely explored in recent years. However, many of these explorations lacked substantial representations of integration practices in Southeast Asia and in-depth analyses of the factors influencing them. To fill this gap, this study investigated how Southeast Asian English language teachers integrated GAI tools across various stages of L2 writing instruction and the challenges they encountered in doing so. Analyzing qualitative data from 14 selected participants across nine Southeast Asian countries, the study found that participants used GAI tools at different stages of L2 writing instruction, primarily to scaffold the development of various writing skills. They commonly used the tools to generate model texts as instructional materials, to facilitate activities for brainstorming and outlining of ideas, drafting, proofreading, and editing of drafts, and to provide immediate corrective feedback. While the participants in this study reported different GAI-integration practices in teaching L2 writing, the practices appeared limited in variety and complexity, reflecting a lack of the requisite technological and pedagogical knowledge and competencies for systematic, transformative GAI-enhanced L2 writing instruction. Implications of the findings for language teacher education and professional development programs are discussed.